The maximum Wi-Fi link speed (PHY rate) a given client can get when connecting to a given AP is the intersection of which speed-increasing factors both devices support (the overlap in the venn diagram of what they support).
Smartphones — even top-of-the-line models — often don't have the power and thermal budget and physical space required to support all of the speed-increasing factors that a top-of-the-line AP, laptop, or desktop Wi-Fi card can support. I've never seen a smartphone that supports 3 spatial streams or more, even though 3x3 APs and laptops started shipping in 2009.
So if one device supports 2 spatial streams but only 20MHz-wide channels, and the other device supports up to 80MHz-wide channels but only 1 spatial stream, the subset that they both support is 1 spatial stream and 20MHz-wide channels.
With 1 spatial stream and 20MHz wide channels and only 802.11n modulation schemes, the max PHY rate you can get is 72.2 Mbps. It seems likely that this is the situation you're in.
The only piece I'm unsure about is your OnePlus 3. It's hard to find reliable information about how many spatial streams it supports. It uses a Qualcomm Snapdragon 820 SoC, and at least one variant of the 820 supports two spatial streams. It could be that your OnePlus 3 uses a different variant that only supports a single spatial stream, or it could be that OnePlus decided to save battery and space by only enabling support for a single spatial stream even though the chip they were using supported 2 (it's very common for system designers to choose not to fully exploit every possible feature of a chip they're using, when the tradeoffs like saving battery and size are good enough).