RRSP return

[telly1]

This may be trivial and I'm missing something obvious here but maybe someone could help me out...

Keeping this pretty straight forward:
Assume $75k income, $0 current RRSP contribution.
For simplicity, my current refund is $0.

If I add $1000 to an RRSP, in Ontario, should I now see a new return of $434.10? $2000 should result in a return of $868.20.
I know the OHIP brackets come into play @ $72.6k.

Right now I'm getting a return of $260 with a $1000 RRSP contribution. As a 'what if', I reduced the amount of foreign taxes paid by ~$6000 and the extra $1000 RRSP contribution produced a tax savings rate of 43.41% as expected.

All of my income (save ~$3000 rental income) is from US employment (foreign income).

Can anyone explain why the difference in tax savings rates?
If say my ON taxes owing were already zero, this might make sense but in fact, before the RRSP deduction, my taxes owing are:
$622.30 federal and
$289.41 ON (before the OHIP premium)
so I would think that a $5 RRSP contribution should still produce a 43.41% return but it doesn't. Even the 1st $5 gives me a return of only 26%.

A return on RRSP contribution of 26% tells me I'm better off not making the contribution and instead investing non-registered and paying the tax bill. Any insight into why this is would be greatly appreciated.