Air Carrier on visual approach to TPA was questioned by ATC regarding apparent no compliance with altitude crossing restrictions over MacDill AFB.

Narrative

ATC cleared us to fly a visual approach to Runway 36R at TPA and to turn over MacDill Air Force Base and maintain 2;600 FT until north of MacDill. I turned abeam the northern edge of MacDill; cut across the corner of the field boundary; continued the turn to a more northerly direction; and descended below 2;600 FT. ATC said we were too low over MacDill and we said we were north of MacDill Runway 4; but still over the field boundary. Then ATC said we were actually 3/4 mile north of MacDill and that we didn't turn over MacDill. We said we did; but always stayed north of the runway. He canceled the clearance and had us climb back to 2;600 FT and turned us back to a 270 heading for a few moments before re-clearing us for a visual approach. That clearance he gave has always; in the past; meant to stay at 2;600 FT until south of the runways which we did. We felt we had complied with his instructions since there wasn't a definite point over MacDill given. I suppose he wanted us to fly further south before turning in to the runway and descending; but if we had; I would still have descended where I did at the north field boundary.