The meaning of an attribute number is dependent on the attribute. Some attributes use an additive value (so 0.1 is +10%, or 110%) while others use a multiplicative value (so 0.1 is 10%, or -90%).
For the MvM .pop files specifically, it looks like they're mostly multiplicative, but quite a few are additive. Going over all the provided .pop files results in these values being used for the "important" attributes, as well as their likely meaning based on value and context:
Attribute Values Meaning
airblast pushback scale 5 mult %
airblast vulnerability multiplier 0.3, 0.4, 0.5, 0.6, 0.7 mult %
critboost on kill 3 add secs
damage bonus 0.05, 0.2, 0.5, 1.2, 1.5 mult %
damage force reduction 0.3, 0.4, 0.5, 0.6, 0.7 mult %
effect bar recharge rate increased 0.1 inverse %*
faster reload rate -0.8, -0.4, 0.2 add %
fire rate bonus 0.3, 0.5, 0.6, 0.75, 1, 2 inverse %*
heal rate bonus 0.1, 5, 10, 100 mult %
move speed bonus 0.5, 1, 1.3, 2 mult %
Projectile speed increased 0.5, 0.65, 1.0 mult %
uber duration bonus -3 add secs
ubercharge rate bonus 0.1, 2, 5 mult %
* An inverse % is when the length something takes to complete is altered, resulting in the opposite effect occuring to the delay between uses. For example a fire rate bonus of 0.5 results in the firing animation happening in half the time, so the result is being able to fire twice as fast.
As you can see there's not much correlation between the attribute wording (e.g. using "bonus vs. "increased") and what the number does. It's a trial-and-error/observational process really.
More info: http://wiki.teamfortress.com/wiki/Template:List_of_item_attributes
