NFPA 1951, 1977, and 1999 certified gear

[somevolunteer]

I'm still in recruit training so I apologize if this is a stupid question. Why is there dual certified gear certified for 1951/1999 and 1951/1977 but not all three?

My volunteer department only provides structural turnout gear but I want to buy a set of lighter weight gear myself. We have a few field fires but I would be using this mostly for MVAs. It seems like the 1951/1999 gear would be the best choice if I can't get gear certified for all three. However, it looks like that the 1951/1977 is a lot cheaper.

Any advice?