It is really then a question of how much of the detail from each entry on the 1911 transcription has been transfered onto the IGI. My experience from looking at parish records suggests that the IGI includes the details that fit into the boxes (so to speak) but there are often additional bits that do not fit and therefore do not get on to IGI.
Conversely there are sometimes assumptions made in the transcriptions which are added by transcribers - usually as individual records.
For example if you look at the marriage of Thomas Clucas and Margaret Cowley (7 Sep 1811) on IGI you get two entries. One is from the 1911 transcripts (I assume) the other is an individually submittted entry with approximate dates of birth added. It now seems these dates are way out - so where did they come from.....
Cheers and thanks