@Lhorn

 It depends on what you are testing, where you are testing and how you are testing. 

If you are evaluating Internet speed you should use the Internet speed test (like OOKLA or SPEEDOF.ME) and perform the test from a PC connectedvia ethernet to the main Router node, or use the built in Deco Speed test.  If you test that same web site from a Windows PC it will be way different that what you will get testing from a cell phone. 
There will also be some speed loss due to link loss between nodes. So if you are in a star config where you have two or three nodes all individually connecting back to the main router node over WiFi, then testing with the same web site and same hardware from each satellite node should be roughly the same.  If you have ethernet backhaul between nodes that will also affect the speed. Of course then the quality of your ethernet cables come into play.
However if your Deco nodes cascade in a chain, from one to another and so on, then testing will get slower as you move to more distant nodes down the chain. 
If you want to test your LAN speped then you will need to test downloading a file from a specific computer on your LAN, preferably connected to the main Router node, using a cell phone to move from node to node. 
And Internet WAN speed testing will always provide a different speed number from second to second. 
Hope this helps