If you're looking at the mtgox.com page, it will show you the high price in USD, but the last price in whatever currency it happened but converted to USD. I don't like it that way (I think they should keep it all consistent, either USD or last currency) but that seems to be the way it works. So it's quite possible for it to show the last price above 30 (if someone bought BTC with Yen, for example) but the high price remains under 30.