Two theories...
1) We know the number of bars is not a precise measure of signal strength. Let's say, for sake of this discussion, that each bar represents 100mV of RF energy received, so 5 bars = 500mV. Person #1 has great reception and is actually getting 700 mV and shows 5 bars. He touches the antennas and drops by 200mV to 500mV, still 5 bars. Person #2 is barely getting 500mV and sees 5 bars. He touches the phone, drops the same 200mV as person #1, but now sees 3 bars (300mV). This could explain why some see the issue and some don't.
2) We know that Apple is trying to do some more sophisticated GSM signal management in iOS4. I believe I also read there may be a software bug in the display of signal strength. So, let's say the bar display is messed up and the perceived loss of signal is not real. BUT, when the false signal display drops to "no service", iOS thinks the signal is gone and disconnects your call. This could explain why dropped calls are being seen, even if it is just a firmware bug.
Yep. That and people not doing any REAL field tests with more careful controls etc. We just have to wait and see what Apple says in their defense. Best case scenario is the signal issues are fixed with an OS update (or even a baseband update), and the yellow screen spots issue is only present in around 0.5% of all devices.