The Kanger has a long 510. The ESP has a short receptacle. Mine does the same. Do NOT crank it down to flush. You will jam the ESP down into the insulator and it won't come back up. My solution... Cut a circle out of a thin piece of leather. Cut a small circle in the middle of it. Use it as a gasket. I'd send you a pic of this pretty patent leather fix it, but I failed to complete the final step: glue it to the SubTank. If you don't, it's likely you'll lose it.
As long as it's firing correctly, don't worry too much about it. Fix it if that gap drives you nuts, ignore it if you're able.