If you were using the same piece of plastic for both pieces, it's likely that some other factor was to blame for the difference in results:
...one was in a hot spot in your oven (or just higher/lower in the cavity or closer to walls or bottom), or on a surface that absorbs heat
...one has a colorant or addition on the surface that darkened/burned more easily than the other
...one was put in at a different time so the temp might not have stabilized or might have been spiking
Plastics aren't all the same though. Some will shrink, some will melt, some will harden and burn, etc, etc. And the temps at which they do their thing can be in a fairly small range so temp differences can matter too.
Transparency sheets are acetate, which may shrink but will also do it unevenly and won't necessarily flatten back out the way the shrink plastics will.
Plastics that shrink (i.e., "shrink plastics"..."Shrinky Dink" is a company which puts out various shrink plastics under the Shrinky Dink brand) are often clear and flexible, and often #6 recycling number (some examples would be clear salad boxes and drinking cups... or some of the foam dishes/cups will shrink as will the bags used for snack chips, etc. If you want to use plastics you find around the house, I'd stick with some of the flat areas of salad boxes, perhaps, etc.)
You can read more about all that on this page of my site:http://glassattic.com/polymer/MISC.htm
(click on the subcategory called *Shrinking Plastics* then read all the info there for the parts that would concern you)