EI Lamination core geometry for 3-phase transformer
In the link you posted, tables 3.10 and 3.11 give figures for the size of this manufacturer's 3 phase cores.

The sizes start off 0.25, 0.375, 0.5, which is linear increments of 0.125, then  it continues 0.562, 0.625 which is now 1/16ths, then back to 1/8ths with 0.875. Once it gets to 1.0, it then continues 1.2, 1.5, 1.8, which looks like it's out of the E12 resistor series, for which the next size would be 2.2, but instead it jumps to 2.4 then 3.6.

I think it's safe to say there is absolutely no coherent series here. It's likely that the company has been asked by specific customers at different times to design particular sized cores, and has then simply put them into the catalogue in size order. There might have been an attempt from time to time by different people to impose some sort of structure on one part of the size range, which never got carried through into a complete revamp of their catalogue. I would not be surprised if other manufacturers matched some of the sizes, and were different in others.

When people try to structure a range like this, they'll tend to use a logarithmic scale. Popular ones are E3, 1.0, 2.2, 4.7, E6 which infills E3 with 1.5, 3.3 and 6.8, and E12 through to E192, which I'm not going to detail here. Another one, which I am trying to commit to memory so I can approximate logs in my head, is E10 which goes 1.0, 1.25, 1.6, 2.0, 2.5, 3.2, 4.0, 5.0, 6.3, and 8.0. The E10 tends to get used for capacitor voltages, whereas the E12 series gets used for the values. No, I don't know why either.