It's also not unusual for Henry to put them together with 'what's on hand'. Very easy to imagine a BB18 being put together in the 1934 model year, with a 34 serial number, and using body that has the 33 features.
For comparison, my sedan, either 33 or 34, whichever you want to call it, has features of both years. Body tub is clearly 33, but with a few 34 features. Hood, grille, dash are clearly 34. It's clearly an original assembly, not put together from parts. Original paint, never a repaint, is on it, front to rear. Frame carries a late 33, or early 34 number, whichever you want to call it.
So, I don't find it very unusual for what you have. Title it either way you want to believe. My suspicion is that you have what would have been sold and titled as a 34, which happens to have a leftover 33 body. JMO