I have heard something from some mechanics but don't know if it is true or false. They say when you are driving on the highway and your A/C is on, the compressor keeps kicking in and out which puts a tremendous load on the serpentine belt and causes the crank pulley bolt to get tighter than the original torque you had applied to it and this makes it harder to loosen the bolt later. They say for example if you had tightened the bolt to 120 lb-ft, the next time you want to open it, it will be relatively tighter as if it has been tightened to 160 lb-ft (numbers are just examples for clarification). I know how A/C compressor works and it constantly kicks in and out during driving but can it really affect the tightness of crank pulley bolt?
There is NO WAY this is the reason the bolt seems tighter upon trying to loosen it, and directly for the reasons jwh20 has stated. There is one more reason why it couldn't happen on most engines.
Since almost every engine out there turns clockwise while it's running (yes, there are a few exceptions, like the Honda D17a series engines), the torque applied to the crankshaft pulley from the belt would be applied counterclockwise. If the pulley were to slip on the hub, it would mean it would be loosening the bolt, not tightening it.
Sounds like old mechanics lore to me. I've never experienced this personally and found that in nearly all cases, corrosion aside, that the bolts in question are easy enough to remove but the removal torque is always greater than the torque spec on installation.
The pulley on either the A/C compressor or the crank pulley should not be moving with respect to the bolt. In many cases it's keyed to prevent the pulley from spinning. But even if not, I doubt that what they've described to you as happening is actually happening.