All I see is that with the previous cables I get much more of a difference. With this cable the difference is practically meaningless for my needs. Unfortunately I have no other way to measure power but with the SA. Maybe I should get some sort of power meter from eBay. If you have any suggestions how could I somehow check on the sig gen I would appreciate it A LOT.
I assume that by difference you mean the difference between the level set on the generator and the level displayed on the SA? If so that makes sense, the more losses in the cable the bigger the difference will be (the lower the reading on the SA in relation to the setting on the generator). Each cable has a certain loss per length, and this loss increases when the frequency increases, there's no way around that. But, the cable doesn't care what amplitude the signal is, if it loses 1dB over 100 meter at 100Mhz it will do that on +20dB and on -140dB. If you keep increasing the output you will eventually start to melt the cable and then the former is no longer true, but your generator is far from able to output that much power
Almost all generators struggle to output the maximum they specify, certainly on the higher frequencies. If you do the same test on 10 MHz the difference could/should be less.
What is your goal? I can't really make any recommendations if I don't know what you are trying to do…