Remember: bars mean nothing. One could theoretically make it so that it's like this:
-90 dBm = 5 bars
-91 dBm = 4 bars
-93 dBm = 3 bars
... and so on ...

Every 3 dBm you increment/decrement means two times the signal you're getting/not getting. So, if Apple did this comparison with dBm, there might be some legitimacy.