Questions about tubes and their subsitites

Hi all.

I use a Cary PH301 phono stage in my system. I have enjoyed this phono pre for many years. Been tube rolling lately as well as swapping various preamps and also some cartridge swapping, trying to find the best combination with what I have. The Cary PH301 uses 5 tubes, 12AU7 buffer, 2 12AX7 for MM and adds 2 12AU7's for MC. I use Sylvania Gold Pin 5751's in the AX7 spot, I have a Sylvania 5814 in the buffer spot (Cary suggests this replacement in the manual). Question one is why is a 5814 considered a replacement for 12AU7 when their values are so far apart? AU7's have gain of less than 20, 5814's are the same gain as 5751, at 70. Why are the 5814's considered to replace 12AU7? 5751 also is 70 but is considered to replace 12AX7, which is at 100. I don't quite understand the hows and whys of this. Any thoughts? Thanks,

Bill