Part of the Furniture
i would say not improved , but more only improved marginally thats mainly due to fine tuning and beamforming as the ( for example ) rt-ac88u does cover a greater area than the rt-ac68u as far as client connectivity goes
The improvement you're seeing is diversity gain with the extra radio on the 88U vs the 68U - that extra radio does help there, and it's symmetric from client to AP. Additionally, a 4-stream AP is always 4-streams, when talking to a 1, 2, or 3 stream client, it's just changes the code interleaving - which can be a pretty nice benefit, esp. in the mid to long range - might actually get an MCS step higher compared to a 3-stream AP.