As noted above that's a historical nickname. Greater Anglia's class 90s and mark 3 rakes were achieving 58323 miles between technical incidents in the year to October 2018, more reliable than any other inter-city fleet in the country, and streets ahead of 17009 miles per technical incident being achieved by LNER's class 91s and mark 4s during the same period.
I'm confused as to how that's relevant to this thread? Grand Central are using class 90s with five mark 4s, not class 91s on load 8, and Hitachi AT-300s will be in the minority on the WCML.
The point being made was that 90s will be faster to 110 than 91s. Plus the 90s are a known quantity on the WCML, rather than introducing a new traction type to the route.
Apologies if I've missed something.