Explain me one thing please.... Shoujo-ai GL 少女愛 (shoujo = young girl(yeah we have other meanings, for example virgin, but different first kanji and there is short Sho...) Ai = 愛 Love (for example, "AI"shiteru "してる" just mean "you suggest, tell someone what you or they/he/she **** to do; " I'm not a teacher so can't explain it properly... So "AI" its love, but Yuri 百合 ( lily ) this flower just , like a symphol for this kind of relationship! Meaning same! Just when Japanese peoples think about GL (shoujo-ai) staff they imagine Yuri (Lily) . Why? Don't wanna explain it, (I wanna, but it will be long story from me) find it yourself. You Will find explanation for sure.
PS sorry for my English, but it really irritating me. I need search shoujo-ai and yuri tags just cuz someone think they're different! And I don't blame mangadex only