It's hard to know what Kelleher's strategy is in respect to certs and the PSAG. My guess is that the reason PSAG is not on their list of accepted expertizing entities is simply because they failed to update their "Conditions of Sale". Currently that document doesn't even directly address the process of extensions except to state that lots with certificates from recognized entities within 7 years of the date of the sale are sold "as is".
In the past, Kelleher most likely granted me extensions on lots with PSAG certs because they want to keep me as a customer, and not for any legal reason. Auction houses are, after all, basically just dealers with a different selling and marketing plan, and they want satisfied customers.
It's no secret that the bane of all auction houses are "bad certs" coming back from extensions, and I only wish we could hear some of the conversations that have gone one between auction house managers and people at the PF and PSE because of these. If the auction houses are finding the PSAG easier to work with, and buyers are willing to accept PSAG certificates, we may see that entity used increasingly.
For those interested, here are the paragraphs relevant to authentication in Kelleher's "Conditions of Sale":
EXHIBITION AND INSPECTION OF LOTS; QUALITY AND AUTHENTICITY
7. (c) Quality. Any lot which a purchaser considers to be incorrectly described may be returned to Kelleher within two weeks of its receipt by such purchaser ("Returning Purchaser"), provided, however, that the same is received by Kelleher within four weeks of the date of the auction; however, Kelleher may, in its discretion, refuse acceptance of such returned lot. If an opinion of a mutually acceptable authority is desired, the period of time within which a lot must be received by Kelleher will be extended in accordance with Condition of Sale 8. In the event that a dispute cannot be resolved by reference to a mutually acceptable authority, and Kelleher thereupon undertakes to re-offer the lot with a description identical to the description disputed, the Returning Purchaser shall be liable for the deficiency, if any, between the proceeds of the sale to the Returning Purchaser and the proceeds of a sale to a subsequent purchaser, as well as for all costs and expenses of the resale, including commission, and all incidental damages.
(g) Lots with existing Certificates from the Philatelic Foundation (PF) or Professional Stamp Experts (PSE), British Philatelic Association (BOA), Royal Philatelic Society of London (RPSL), Vincent Graves Green Foundation (VGG) or any other expertizing authority as recognized by the International Association of Philatelic Experts (AIEP), for items within their specialty and dated within the last seven (7) years from the date of sale, will be offered and sold on an "as-is" basis and as so stated on the certificate. Such lots may not be returned for any reason, including but not limited to a contrary verbal or written opinion or certificate.