(All information can be found at the CS:GO Weapon Finishes Workshop Guide, which can be found here)
The reason you're seeing variants in your two knifes (despite them being the same skin) is due to CS:GO's randomization of skin offsets/scratches. What the game does is move the skin down/left/in any direction, thus making it look different despite being the same thing.
Another thing that gets randomized is the scratches. Depending on the quality, you'll see more scratches, but it does not dictate where these scratch marks will be as it'll be different for each gun.
If you're thinking that they're completely different weapons, it may be due to what the skin looks like after it's been shifted (ie. The bottom and top half of the 'butt'; back handle look completely different and they look like two different patterns when looking at it... Or your eyes may just be playing tricks on you.
Note: Not all skins have skin offsets. For example:
A MAG-7 | Irradiated Alert is a custom skinned weapon, yet the design always stays around the same place (still moves, but it's hard to see with a naked eye), while a P250 | Contamination for example could have the entire texture moved, whereas a MP7 | Anodized Navy would never have offsets (apart from the scratch marks) as it is a solid-color skin (same as using the paint bucket).
Remember, a variant is never a new design of a weapon it is only the difference in the skin's offset and weapon scratch locations/amounts.