I’m a little surprised.
You felt that the response given to your gift was not as good as the responses given to other, similar gifts. You then inquired as to the difference with the lady receiving the gifts.
And she didn’t slap you upside the head? Missed a golden opportunity, there. I can see making a gentle comment if she failed to thank you at all, but saying something to her because you didn’t think her “thank you” was effusive enough or excited enough is just… I don’t have the words, but I’d be pissed about it if I were in her shoes.
If she thanked you politely and sincerely for your gift, why is her behavior being called into question here? And, really, why are you comparing her responses to various gifts anyway? Do you win a prize if you get the most effusive response?
I’m inclined to believe her explanation (and a little horrified you felt compelled to demand one). I know that I tend to be more effusive when presented with gifts I didn’t expect than ones I was expecting. Maybe she wanted her picture taken with the gifts from the other fellas because she wanted photographic evidence they actually remembered her birthday and appeared with an appropriate gift since that’s never happened before and never will again. (This is why I have a picture of myself with flowers my brother gave me - it remains, to this day, the only event my brother has managed to remember and appear at with appropriate gift since he got too old for my mother to prod him about it.) Or maybe she was extra-effusive with the other fellas to encourage them in this behavior in the future - and knew you didn’t require the extra encouragement.
She was probably counting on you to be gracious enough not to make a big deal about the difference, since she apparently considered you a gentleman.
You’ve mentioned that she did some other things you’re not happy about, but you seem fixated on this flower thing. I have to tell you that I’m not sure you’re in the right to be incensed about it, frankly. One shouldn’t give a gift in the expectation of reward (like effusive thanks and gushing).
On rereading this response is a little harsh, but I stand by the principle. Ideally, every gift giver should be identically treated, but that’s rarely the case, and being hurt about it is essentially being overly thin-skinned (in my opinion only, YMMV). Commenting to her about it is actually rude, also in my opinion only, YMMV.