wi fi - Why do two smartphones connected to the same WiFi network show different speeds (one displays the link speed as 135 Mbps and other only 72 Mbps)?

I have two Android phones:

1. 
   

   Xiaomi Redmi Note 3

   
   



2. 
   

   Acer Liquid Z630s

   
   

On Redmi Note 3, the link speed is only 72 Mbps and the download speed hardly crosses 50 Mbps. Whereas on Acer Z630s, the link speed is shown as 135 Mbps and the download speed has reached 90 Mbps on Speedtest.

On average, download speed on Acer has been 6 - 9 MB/s whereas on Xiaomi it has been only 3 - 5 MB/s. This is quite surprising as Redmi Note 3 has a Snapdragon chipset and also supports dual band whereas Acer has a Mediatek chipset and supports only a single band (on using 5 GHz mode on Redmi Note 3, no WiFI signal appears).

What could be the problem and how can I improve the WiFi capacity of Redmi Note 3?