The speed is coming from the ISP and the modem/router. Not the tablet. You can connect multiple devices and have them all get a different speed to show up. The more devices on the connection, the more changes you will see.
Plus, if your connection fluctuates a lot, this would cause it even more. That would be different for different devices. Some more easily adjust to changes in speed, others do not.
Also, it is completely possible that your ISP has put limitations on the speeds you can get at certain times of day. I had that problem myself. Had to catch them at it first and then get them to resolve it. I am paying for that speed all the time, not just certain times of the day.
Lastly, your modem/router may have limitations within its settings for how many devices can run before it will lower the speed. Or even certain devices, or certain types of devices, that it will change the access/speed for.
OH, and if your ISP is like many, they may also be slowing down your access depending on what you are doing and on how much data you are using.