sure, seeing as you ask so nicely ;-)
You're not looking at LPQS if you're getting a number - Landing Page Quality Score is only rated as Poor or No Problems (well, as much as we can see anyway).
What you're describing is the overall Quality Score of two individual keywords.
The most likely reason for your differing QS between the KWs will be that your singular has a CTR which is quite high when compared to your competitors, and your plural has a CTR which is quite low when compared to your competitors.
It doesn't matter if the 2 CTRs are almost the same as each other as the key here is that the CTRs are comparable to competitors bidding on the same keyword.
On top of this, other matters such as historical performance of the two different KWs can affect things - i.e. sometimes when first starting out with a new KW you can get different QS for the singular and plural versions right at the beginning - this is because historically they have different CTR / QS levels for other bidders as they are two separate KWs
>>And, if you ask why the heck I am advertising for a term that I'm #1 on? Don't ask - I like to.
Agree with you on this one though, I always advertise on KWs that I'm also #1 for organics