The original French sound is, to the best of my knowledge, somewhere between an unglided version of a long A and the short e that y'all seem to prefer. It did enter English rather late to have undergone the Great Vowel Shift*. However, the brick and mortar OED seems to indicate that the sound is the same as the vowel sound in thief. So what's y'all's excuse?

*That's Shift not Movement. Nothing to do with Finno-Ugric.