Before 1969 there is no surname shown for the child on a birth registration, so the surnames that appear in the index are those of the parent(s) named on the entry, one or both, depending on their marital status.
This is an odd one and I can't immediately think of a combination of names that would account for the way it appears, and there is no indication of any later correction.
All you can say is that the names Mann and Howard are on there, and also the mother is or has been married (or is claiming to be) because there is a maiden name of Young also shown.
It is quite normal to get different results between the old printed indexes, as shown on FreeBMD, and the new GRO on-line system because they don't use the same indexing rules.