Because PDSCH interference is different than SSB interference.
PDSCH is related to scheduling.
SSB interference is related to PCI mod4 in NR.
PDSCH interference is about overlaping between cells and about both cells scheduling in same time same PRB same location.
SSB interference is about cells overlaping and same PCI mod4.

Allow me to doubt the correctness. If it is interference is not always 8 or 14 dB. It fluctuates in time and so difference should fluctuate in time not be always 8 or 14 dB. And excuse me for asking, if you can calculate why to measure? I think those are measurements not calculations.

Or at least should be measurements not calculations.
I mean you calculate your signal from your cell because you know its values based on RS. But interference you have to measure it, there’s no way to “calculate” it. So here is goes SINR signal interference noise ratio.