An authoritative list kept so that entries can be relied on. A registry's worth is that someone maintains it.
Built on register, from Late Latin registrum, from regerere (to carry back, to record): re- (back) + gerere (to carry); a registry is only as good as the carrying-back it is trusted to keep faithfully.
An authoritative list kept so that entries can be relied on. The root's carrying-back only means anything if someone keeps doing it — a registry's worth is entirely in its maintenance.