Engines at full strength are generally very good, so the programmer has to come up with ways to make it play worse (lower levels than max).
One way is to restrict its thinking time. 1 second per move for a very low setting, 5 seconds per move for a better setting, etc. However 5 seconds on your phone's processor can't get as far as 5 seconds on your PC.
I am just a beginner but I play a lot of chess against the computer. Level 5 on my phone is fairly easy and I win 70% of the time. Level 6 kills me and I've only won a few times.
Playing level 5 on my PC , however, is a different matter and I get cleaned up nearly every game. Why is there such a difference? Shouldn't they play at the same level?