The thing is, while both deal with time, they are very different abilities.
Time Stop alright halts the flow of time, it simply stops and nothing moves. It freezes the present moment for a prolonged amount of time.
Time Slow, meanwhile, is a distortion of the flow of time. An application of Time Manipulation. The present is stretched, making seconds last longer than seconds, making everyone other than yourself slow to a crawl. The reverse of Time Slow is Time Acceleration, and in there you boost the speed of your own time, while everyone else is normal. In this case the results are much the same, but for different reasons. Both boost your speed through Time-warping means.
Another example of Time Manipulation is Time Reversal, which is hitting the rewind button on the flow of time.
Though both deal with time, Time Stop and Time Manipulation are completely different abilities. The former is just one application of the later, and not necessarily means that resisting it will allow you to resist every other applications, specially when they work in similar ways.
To use an analogy, take these two abilities: Atomic Destruction and Matter Manipulation.
Let's say your character can endure Atomic Destruction, he can resist attacks which destroy the atoms of normal objects. Does this mean that he can resist being atomically disassembled by Molecule Man? Of course not, it is a completely different ability than Atomic Destruction, only bearing a surface level similarity in that both deal with atoms.