I've heard from my match who did a Tier 1 segment match and I'm not on that list, although I am on the My Heritage one (and triangulate). In fact she and her other relatives who match with me (and triangulate) on My Heritage have much lower shared cMs on Gedmatch.
Very odd. 
Gadget
Strange, are you definitely checking the right chromosome

. I took the plunge myself and have confirmed some shared matches on Ancestry do share the same segments, but unfortunately only a tiny % of Ancestry users are on there. Also a tiny % of GEDMATCH users have GEDCOM trees so it's hard to get a bearing on who or where most people are.
One thing I do think is the case, is the GEDMATCH segment match hasn't yet been updated with the new Q-Match matching algorithm - the one to many tools both are and kits that match one or two tools are, but the segment search still seems to use the old matching method, which is confusing. The One to One Q-Matching is a currently a premium feature - the free One to One uses the old match system which doesn't match the other free tools, which is again, confusing! Haven't yet tried the triangulate tools, and haven't got enough close family members tested to try the phasing yet.