I don't think Webley invested in new barrel making machinery, just kept churning out on their old machines. So the barrels are accurate enough for the farmyard ranges and pellets available at the time. 1" at 30m is accurate for the time, and for 20m thats a solid hit on farmyard critters. Why would you need more when the farmyard dog would be onto anything in a flash that was hit?

Theoben started with Webley barrels, but fast switched to Anschutz. The latter had newer barrel making machines, so the barrels were more accurate.

And then time and progress, and cheap Jap scopes arrived and the consumer demanded more. So the HW77 arrived!
I think for the Omega, Webley made some changes, as it was already changed. Ownership and maybe had some investment???