Seems dodgy. Are they fakes pretending to be Silverline?
Sadly you're right. It's a poor attitude really, and false economy. I haven't used the Silverline ones (and won't) but if they're anything like the set of Irwin TiN bits I've got (they were given to me- don't buy them, they're junk), I'd expect to be replacing them every few months at best. That's the Dormer set paid for in under a year.
I only tried thier sds masonary drill bits, and they were perfectly good. I dont do a lot of drilling so, i cant really comment on the long term quality.
Dormer are a great drill bit, but in the Throw Away World we all live in these days, unless you're using drill bits on a daily basis and prefer to personally sharpen your bits as they go blunt, the silverline will always be picked over the Dormer
Have you tried the Heller TiN-coated or cobalt bits out of interest? I've generally found Heller alright for the money but haven't tried either of those yet.