Well mobs insta-summon the moment you get out of range. Perhaps when you zone, gate, or even die, the server automatically detects that you are out of range and insta-summons you.
This of course, means that as soon as you zone, or start to zone, you'll be summoned. Likely this happens before the client can communicate with the server and say "HEY! I'M NOT IN THAT ZONE! DON'T SUMMON ME!"
Just a theory...
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