The solicitation instructed offerors to demonstrate the "relevance" of their Past Performance references/past contract work. The solicitation defined levels of "relevance" by doing X number of units of a certain service "on a monthly basis." Offeror "A" submitted a proposal that said it did 3,000 units per month under a past contract. However, when the evaluators looked at the PPQ submitted by the PP POC, the PPQ said "A" did only 200 units per month.

In determining the "relevance" of this PP, should the evaluators base their decision on what the Offeror said in their proposal, or what the PPQ/third party said?

I wouldn't do either. I would clarify with the offeror. FAR 15.306(a)(2):

If award will be made without conducting discussions, offerors may be given the opportunity to clarify certain aspects of proposals (e.g., the relevance of an offeror’s past performance information and adverse past performance information to which the offeror has not previously had an opportunity to respond) or to resolve minor or clerical errors.

Don's advice makes sense, if there is a possibility of award without discussions to that offeror (based on a "good" response), or if the response to the clarification will provide a basis for deciding not to award to that offeror (based on a "bad" response).

If this offeror is otherwise among the lowest rated for the other factors, and higher priced, and past performance is a factor of lesser relative importance than other factors, and you can make an award without discussions decision for another offeror, do that.

Or, if you need discussions, see if your decision of who is included or not included in the competitive range can be made based on the other factors. If the offeror cannot make the competitive range because of other factors and regardless of whether it gives a "good" or "bad" response, then you may exclude that offeror from the competitive range without resolving your dilemma. The information isn't really adverse unless you rely on it to the offeror's detriment.

I want to point out that you can seek clarification even if you intend to conduct discussions and say so in your RFP. FAR 15.306(a) is intended to make clear that you can seek clarification even if you intend to award without discussions. It should not be interpreted to mean that you can seek clarification only if you plan to award without discussions.