Exceeding Max Heart Rate

Hi

According to the 220 minus my age calculation, my max rate should be 181.

When I come back from a hard run session and review my stats from my Garmin, often my max heart rate reads at between 175 and 182 (average 150/155).

I'm obviously concerned about going over my max but although i'm fit, find it hard to max below this on hard sessions.

Any thoughts/advice.

Thanks

James