Please tell me, why do the two models 5M1270ZF256I5N and 5M1270ZF256C4N appear on the same chip?

Hi,
you are crossposting the question a third time today, but it's still not clear. I'm answering to the old question because the title allows at least a guess of what you are asking.

As far as I understand the question, the answer is quite simple. You have a chip that achieves fastest speed grade 4 in consumer temperature range (0 to 85 °C). It can also be used in industrial temperature range (-40 to 100 °C) but is only achieving slower speed grade 5. This is most likely due to the fact that the FPGA becomes slower at high temperatures.

Regards

Frank