Total % contracted is partially based on corporation production %.
For example, if hiring % changes, then contracts may appear to be at one % level, but then change later as the hiring % also changes.
Another example is for corporation welfare changing, and then the % contracted could also change.
Generally speaking, the system was made so that common market auto-contracts were not really designed to be "fixed" amounts indefinitely, unless there are nearly no changes between the two contracting sides.