The 1841 census records where people are on census night (6 June). The only information given about place of birth is whether they were born in the county where they are on that date, or not.
So in 1841 Christopher is in Diseworth, and is recorded as having been born in Leicestershire. We don't know what would have been named as his place of birth.
In 1851 he doesn't appear to be living with family. Perhaps he wasn't sure where he was born, and there was no one around to ask at the time. Or perhaps he was sure that it was Diseworth, because he had grown up there, and didn't know any different.
It isn't unusual to find people naming different birthplaces in different censuses, but generally the places they name aren't far from one another.
The birth certificate you mention is a baptism record, which doesn't necessarily show place of birth, only where the family are living at the time the child is baptised. The place where the family live is given as Shepshed, but perhaps Christopher was born at Diseworth, and the Gents have moved since then - or maybe Sarah went to her mother's house in Diseworth to have the baby, but didn't actually live there.
