> taking mine to the same areas on the same carrier and doing a comparison

Unfortunately I don't think it's that simple. I've seen one phone simultaneously show significantly different numbers of bars for two SIMs installed in it for the same exact network and operator. After a while they become similar... then differ again... etc.

I have no clue how to explain it yet, but what I do know is that it literally makes no sense with a naive model of how these work, whether you try to explain it as reception or deception.

The phone selects a RAT (radio access technology) and frequency for each SIM slot.

After selecting, each SIM slot is subject to inter freq / inter RAT reselection / handover.

Both are controlled by messages received from the tower (e.g. on 4GLTE, for reselection, System Information messages), though there is an additional constraint: what's supported by/enabled in the phone.

Perhaps one SIM slot was in the connected state and the other was in the idle state at one point. So the reselection logic applied for one and the handover logic applied for the other. There is for example a problem called ping pong handover. Once a phone is switched to a different frequency or RAT, the tower may have the phone be sort of stuck in the new frequency, until the conditions of the previous RAT or frequency improve substantially, in order to prevent the phone being like a ping pong ball between the two. This frees resources that would otherwise be spent on repeated handover-related messages.

Each frequency has its own signal strength (free space path loss, transmit power, one frequency might be on one tower and another might be on another, etc).

This is usually for a good reason - dual sim phones are almost always “DSDS”, or “Dual SIM Dual Standby”. The secondary SIM, because it doesn’t need to make a data connection, parks itself on the lowest-frequency (and therefore usually lowest-bandwidth) connection it can find. Meanwhile, your data-connected SIM is busy trying to stream a video or upload your photos, so it’s using a higher-frequency + higher bandwidth connection, resulting in a lower signal strength.

I also have multiple SIMs set to the same network, with sometimes a different amount of bars shown.

I guess the bars aren’t realtime but updates every x seconds? I summed no malice.

> I have no clue how to explain it yet

Android is quiet lazy searching for towers.