GPU operate in a "batch" mode.  They start hashing and attempt so many hashes before stopping to return results.  The higher the intensity the more hashes the card attempts in one batch but also the longer the batch takes.  The card only returns results at the end of the batch.  The higher the intensity (and longer the batch) the more shares which are computed but never "delivered" before the block changes and they become stale.  On the other hand the lower the intensity the more of the GPU time spent setting up and taking down batches which means a lower raw hashrate

You get paid for shares (or work) accepted so that is what you should be looking to maximize.  It is possible the accepted rate is still higher even with a higher rejected rate because the gross (raw) hashrate is higher.

Simplified example
100 shares gross w/ 1% rejected = 99 shares accepted
120 shares gross w/ 10% rejected = 102 shares accepted.

The later is more profitable.  Since you get "paid" for shares accepted that is the number you want to maximize.