There are 2 possibilities

Location of Data:

CDs are read at a constant linear rate. This means that at the read head the CD must travel over it at a constant speed. Data on a CD has the same size per bit on the inside and outside, meaning that there is more data per revolution on the outside of the disc. Therefore, to provide a constant speed of data input over the reading head, the disc spins slower when reading the outside.

Because of this, if the game needs to constantly read data that is near the outside of the disk, it will spin slower

  This would explain different rotational speed for a single disk

Read Speed:

CDs and DVDs are read at different speeds

For the Playstation2 these are the specs:

24x speed CD-ROM [3.6 MB/s], 4x speed DVD-ROM [5.28 MB/s]

24x speed CD-ROM can typically spin at 5k to 12k rpm

4x speed DVD can typically spin at 2.2k to 6k rpm

Source 1 Source 2 PS2 Spec Drive Speed Source

There are 2 possibilities

Location of Data:

CDs and DVDs from this period were usually still read at a constant linear rate. This means that at the read head the CD/DVD must travel over it at a constant speed. Data on a CD/DVD has the same size per bit on the inside and outside, meaning that there is more data per revolution on the outside of the disc. Therefore, to provide a constant speed of data input over the reading head, the disc spins slower when reading the outside.

(Later, faster drives switch to a constant angular rate which meant they always spin at the same speed, and the data input speed changes depending on which part of the disk is being read)

Because of this, if the game needs to constantly read data that is near the outside of the disk, it will spin slower. This would explain different rotational speed for a single disk

Read Speed:

CDs and DVDs are read at different speeds

For the Playstation2 these are the specs:

24x speed CD-ROM [3.6 MB/s], 4x speed DVD-ROM [5.28 MB/s]

24x speed CD-ROM can typically spin at 5k to 12k rpm

4x speed DVD can typically spin at 2.2k to 6k rpm

This is probably why you noticed the CD ones spinning faster

Source 1 Source 2 PS2 Spec Drive Speed Source

There are 2 possibilities

Read Speed:

Different CDs and players have different read / write speeds. Quite often the player will adjust it's speed depending on the CD. However this is usually only during write so this doesn't seem to be the reason here, unless, the disk has issues being read at the highest player speed, in which case it may slow down to get a more reliably read the data.

As these are all playstation CDs for playstations to read, this is unlikely.

Location of Data:

CDs are read at a constant linear rate. This means that at the read head the CD must travel over it at a constant speed. Data on a CD has the same size per bit on the inside and outside, meaning that there is more data per revolution on the outside of the disc. Therefore, to provide a constant speed of data input over the reading head, the disc spins slower when reading the outside.

Because of this, if the game needs to constantly read data that is near the outside of the disk, it will spin slower

It's most likely that this is the reason why some seemed to spin slower than others

Source 1 Source 2

There are 2 possibilities

Location of Data:

CDs are read at a constant linear rate. This means that at the read head the CD must travel over it at a constant speed. Data on a CD has the same size per bit on the inside and outside, meaning that there is more data per revolution on the outside of the disc. Therefore, to provide a constant speed of data input over the reading head, the disc spins slower when reading the outside.

Because of this, if the game needs to constantly read data that is near the outside of the disk, it will spin slower

This would explain different rotational speed for a single disk

Read Speed:

CDs and DVDs are read at different speeds

For the Playstation2 these are the specs:

24x speed CD-ROM [3.6 MB/s], 4x speed DVD-ROM [5.28 MB/s]

24x speed CD-ROM can typically spin at 5k to 12k rpm

4x speed DVD can typically spin at 2.2k to 6k rpm

Source 1 Source 2 PS2 Spec Drive Speed Source