According to the following article on ANN (dated 20 March, 2015, more than 2 weeks before the anime airs), it was confirmed that Triage X was listed at 10 episodes:
A flyer at the AnimeJapan 2015 event's business day lists the television anime of Shoji Sato's Triage X manga as 10 episodes long, along with an original video animation (OVA) episode.
I also googled with queries such as
トリアージX 話数 and
トリアージX 全12話 (the second one is an attempt to dig out any article claiming 12 episodes), limited to articles posted before April, but all Japanese websites list the series at 10 episodes.
Since there is no evidence that the wrong information had been posted elsewhere on the Internet, it's probably due to MyAnimeList populating the wrong information, or a lapse in your memory.