Time/Temp ruler question

I use the ruler for time and temp, just up to temperature for tender cuts, fish etc.
I am a bit puzzled about the pasteurisation times, though
-for instance, a slab 40 mm thick takes 2:30 to come up to bath temp. This is largely unaffected by the bath temp. But at 60 C the pasteurisation time is only 2:00, even though at 2:00 the meat has theoretically not even reached temperature .

Therefore I was assuming that the pasteurisation times are after the meat has reached bath temp all through.

But if so, then why does thickness come into play? Once the meat has reached the desired temp all through, thickness should make no difference?

Thanks for any info.

Nick