Then they haven't watched him play this calendar year (or at least, made their minds up before then and haven't checked to see if they're still accurate - which, let's face it, is extremely common; though I'd suggest still wrong in this case, Zach has never lacked for physicality, he's just no monster, and struggled initially thinking that he was more physical than he actually was).
As for Mercer Vs Simmonds; for me, Simmonds is a 7 who can cover 6, and just about 8, Mercer is a 6/8 who can just about cover 7.
Exeter though, seem to have an innate distrust of putting players in their position of best fit (Slade is an IC all day, every day; Nowell would be better at OC or FB, no-one else would look at a backrow of Simmonds, Ewers and Armand and decide that SS is the most obvious 8). Against this of course, LCD does look to the manor born at hooker, whilst they did spot that Devoto had to be a centre...