IMO, it is two completely different things...

The seller pays the Realtor a commission for doing the job of listing and/or showing his property in an effort to get it sold.  Any credits given by the seller are negotiated concessions, like giving up the stove or curtains.  Because it is a dollar amount and not a "thing" like curtains it must be disclosed as having a sum certain value.  If a realtor gives the buyer a credit, it is a "gift" and the realtor is subject to his own issues (like paying the tax on that income and then writing off the credit as a business expense).  That separation (from seller and Realtor) is enough to make it a different kind of credit.

The lender usually wants to know about seller credits because it shows what type of interest in the transaction the buyer is putting in.  There are specific limits to how much credit can be given by a seller because statistically, there are higher default ratios on those types of transactions.  Afterall, if a buyer does not put a penny down and only uses seller concessions, they stand to lose nothing if they default.  The regulation is tighter between sellers and buyers because they are parties to the transaction (both sign the HUD-1).  The realtor is ancillary to the transaction.  They normally sign nothing for the lender.

Note: I am a MA attorney and nothing in this post is to beconstrued as advise.  No attorney-client privilege is created herein.  This is an opinion post only and only pertains as it applies to MA law and MA conveyancing practices.