My dd is 13yo, and has done this with many words over the years, although these days mostly with less common words. As a toddler, she started learning to read at the same time she was starting to speak. She went on to read prodigious amounts ever since and has absorbed a large fraction of her vocabulary through reading. I chalk mispronunciations up to that, combined with the English Language and its annoying spelling/pronunciation irregularities. She's great at spelling, though, without any explicit effort, no doubt due in part to all the reading she has done.