@AlonIsrael 

Hey

I dont speak for TP Link so this is only what I have observed in use.  

This appears to be a selective algorithm in play, namely the 3rd node will try primarily to connect to the Main Deco at first preference.  Bear in mind each "hop" via another node halfs the speed each time due to the nature of WiFi communications.   Its likely the link between node 3 and 1 is of moderate signal, the node between 3 and 2 is good signal., however the additional hop is going to reduce the performance to a point where there is no difference. 

Say 1 connected to 2 at 300mbps... 2 to 3 at half that (150).. its arguably better that 3 connects directly to 1 at 160mbps using 2.4ghz and will choose that.   This has the advantage of reducing the load on 2.  Remember the more hops, the more load overall, you literally double the traffic each time.  

In my testing I was able to recreate this by moving nodes and blocking signal (yes I was curious that day), direct to node 1 from 3 was faster than via node 2.   Granted this may not be the case for everyone all the time, but it does show that the Deco uses some intelligence to make this decision.