As a native speaker I'm with you. Always seemed pretty obvious to me too, especially above 5 with six-ty seven-ty etc. To be fair though, Chinese says/writes 15

As a native speaker I'm with you. Always seemed pretty obvious to me too, especially above 5 with six-ty seven-ty etc. To be fair though, Chinese says/writes 15 as ten-five which is much more obvious than the difference between five-ty and five-teen. It's interesting in spoken English the emphasis flips between first to second syllable though.