Most people who've ever bowled know there are 10 frames in a game of bowling. Add the knowledge of bowling's perfect score, 300, and it stands to reason 10 strikes in a row is a perfect 300. However, this is not the case.
You Need Two More
As detailed in this article on bowling scoring, the tenth frame consists of at least two and and possibly three shots.
A strike counts as 10 plus the sum of the next two shots. So, if we only had one chance to throw a strike in the 10th frame, where are those next two shots? If the 10th frame was treated the same as each of the previous frames, there would be no next two shots, and the maximum score would actually be 270 (30 in each of the first eight frames, 20 in the ninth (because only one shot was thrown on top of that strike), totaling 260, plus 10 in the 10th frame).
That's why we have the fill balls. These shots give bowlers a chance to complete their scores on strikes and spares in the 10th frame. If you throw a spare, you get one more shot to complete the score of the spare. If you throw a strike, you get two more shots to complete the score of the strike.
12 Strikes to 300
If you start a game with 10 strikes in a row, you're doing pretty well for yourself, but you haven't reached perfection yet. That 10th strike is only worth 10 pins until you throw two more shots. Throw two gutter balls and you'll finish with 270, but throw two strikes - a total of 12 - and there's your 300.