Isn't Dragonscale meant to give a higher Armour Rating than Glass? I've noticed that my Legendary Dragonscale Armour gives me a rating of 171, but my Legendary Glass is rated at 173. Glass Gauntlets are a few higher as well. No so Boots and Helmet though, the Dragonscale is rated higher there.

Seems a bit odd to me.

Dragonscale has better base ratings for every piece according to UESP.
I presume you've improved them with smithing. Was your smithing the same rating each time you improved the items? Don't forget temporary boosts from enchanted items and potions.

I improved all the Dragonscale at the same time, long after doing the Glass. (Maybe I did have some Smithing perked item on when I did the Glass Armour and Gauntlets. I don't usually bother, but maybe I did that time. That must be it. Ta.)

There are like 23 (or something like that) levels of Legendary you can get with the potion, item smithing boost. Your Glass may be a higher level of Legendary than your Dragon Scale.

15 levels is what I read. Either way, "legendary" does not necessarily mean "best possible". You may be able to ratchet up the stats further with more fortify-smithing items/potions and smithing perks.