IFR pilot enroute at 6000 interpreted Controller's clearance; 'cleared to ZZZ airport'; as simply a confirmation of his/her destination; rather that a direct to the airport change in routing.

Narrative

Departed VFR; received an IFR clearance in the air from Approach to fly as filed and climb to 6000. While enroute; I was told to change to Center frequency. I checked in with Center at 6000 FT. Approximately XA25 I was told by Center I was cleared to my destination and at XA31 I was told by Center that I was cleared to the Airport. About 10 minutes later Center asked me if I was on course and that he had told me twice to go to my destination. I said that I was flying the route as filed and would like to continue as filed. The Center Controller cleared me to continue on my filed route. After I was sure the frequency was not congested I called Center back and told him that I was continuing my filed route because I never heard him mention that I was cleared direct to the airport. I only heard him say I was cleared to the airport and assumed he was just clearing me to my destination. In hindsight I might have questioned if he meant for me to go direct from my present position to the airport. Because I never heard the words 'cleared direct' or 'cleared present position direct' I did not have a reason to assume he meant for me to go away from my original clearance direct to my destination.