If we had the answer to that question nobody would post here
You get the small number that is a issue with the router or pc driver software but most are interference.
First there is a difference between signal level and signal quality. You can have great signal level but poor quality because of interference. Many adapters do not distinguish between the two.
Also using software to see other SSID and their levels ASSUMES all signals are 802.11 wireless. Many other devices share the frequencies. You can easily have a cordless phone (or a neighbor might) and whenever its used it wipes out a number of channels. The PC nic is not designed to see this signal so it assumes it does not exist.
Another new problem is that routers have decided to take it upon themselves to try to locate a channel that is has fewer people using it. So now they jump all over the place which in someways makes the issue worse since you may check and see nobody but then 5 minutes later their router decides to use your channel.
Best option is to try to use the 5G bands. There are more of them and they tend to be blocked by walls easier (maybe bad if they are YOUR walls you NEED to pass though).
Bottom line what you are seeing likely is interference and the router and your pc are renegotiating the parameters of the connection.
My wireless connection and internet speed is excellent regardless of distance from the router in my apartment, and regardless of the device connecting. However, occasionally for, say, 30 seconds, there is no internet speed at all (diagnostic tests show it goes down to 0.01 mb/s for a while then back up to full speed). More recently, instead of just the speed going down, the network connection gets lost and then comes back.
This sounds like an network interference problem to me (yes?). I switched channels to one that did not have any of the like 50 visible networks on it. The problem remained. I used inSSIDer to check the amplitude on neighboring networks, and saw that the next strongest network after mine was overlapping my signal and occasionally was stronger. Is this what would cause temporary network outage/internet outage? I changed channels ago so there was minimal overlap with that network, and my internet has been working great for about an hour. If this didn't fix it, any ideas on what could be the problem?