I would have responded with "Callsign - Maintaining 2000" if that's what you were doing... But I'm not sure exactly what you've said even here in writing...
...passed 2000 so I added "and now at 2000"
So were you level or still on descent? Maybe this was the problem, he wanted you at 2000 but you descended lower? But as gc4 said we use +/- 200ft on your Mode C if your transponder read out is outside that tolerance we'd ask you to Squawk mode A only.