I have an Osteopermum Tresco Purple. By all accounts it should grow to a height of 30cm/12". Mine, that I planted in spring, has grown to about 60cm/24", about 100cm in with and still flowering at the end of November and has been a marvellous plant. I have the label that came with the plant and it is definitely the plant in question. Is it normal for this plant to grow into such a large bush? I have attached a photo of it when it was smaller in early summer.

It would be more relevant to your inquiry to post a current photo. Just some guesses...

When I grew them as a groundcover in my yard, they were never taller than about 12 inches but... Every website I read has a different size, ranging from 6 inches up to 3 ft. tall. It could be yours is just trying to stay above the competition (they do scramble over top of other plants) or its really really happy.