I guess it depends on who did the recording of the birth time. Chances are your mom was kind of busy around then and by the time she looked at the clock half an hour could have slipped by. The same with whoever else had the form and pen in hand. If there was any guesswork or rounding off involved ... you could have wound up with this discrepancy. (If there was a doctor involved, for example, he may have just looked at his watch as he was filling out the "paperwork" and written down that time, thinking it would be close enough. That could certainly make a half-hour's difference.)
Try noting the transits to your Ascendant and see when you start really feeling or seeing their effects. If you find things are triggered around the degree of a theoretical Gemini Ascendant ... go ahead and adjust your chart back -- especially if that continues to "make sense" as you said.
If however, that Cancer Ascendant proves to the the sensitive point in your chart, then you'll just have to give a lot more weight to your Moon / Venus in 3rd investment. Where's Mercury in relation to all this? Could that maybe explain some things, too?