This has happened to me twice now, so I thought I would share it.
Forgive me if I am pointing out the obvious, but...it wasn't to me!
I had found a marriage on the IGI for they year 1862, and when I searched for it at SP it wasn't there. My year search was 1860-1863. Nothing. So I took out the years and got a match.
The reason? Although the marriage DID appear to have taken place in 1862, via the registration details, it did not get officially registered until 1873. And the indexes at SP are ordered by registration date, NOT date of the marriage, whereas the IGI lists the date of the actual marriage.
Just thought I would share, in case it might help someone.
Nina