This quote humorously plays with the idea of forgetting someone's birthday but still acknowledging that it wasn't entirely forgotten. It suggests that the intention to remember was there, even if the communication about it was overlooked.
The quote underscores the lightheartedness that can be found in human forgetfulness. It implies that the speaker intended to remember and acknowledge the birthday but failed to convey their awareness.
In essence, the quote reflects the dynamics of relationships and the little quirks that can lead to amusing situations. It's a playful way to admit to a lapse in memory while still letting the other person know that their special day was on the speaker's mind.