Ah, thanks for the explanation, I know the reason then. I have 16 Gb of RAM, so EE expects that 20% (which is 3,2 Gb) should be free, and 2 Gb are less than 3,2. I think this scheme should be improved somehow. Maybe take into account the size of the currently edited file, because, obviously, memory requirements for 20 Kb file and 100 Mb file would be different.
In general, I must say, the description of the new Undo behavior is quite confusing. I simply did not understood how all this worked, until you described it now. And description of the options in the help is not of much use either:
Maximum Memory Size text box
Specifies the maximum memory size used in percentage.