Questions about Cell Strength vs Quality Boron BRN402 vs BRN404


Our customer is delivering units to their end customers and in some cases we are seeing low cell signal strength (CSS) or low cell signal quality (CSQ). I have no idea of the physical location of the installations.
Can anyone provide any information about the difference between cell signal strength and cell quality? Obviously we would like them all to close to 100% but how likely is that?
When we bring units online and do testing at our factory, in many cases I see a lot of the devices connect to AT&T. If I recall if the unit connected to AT&T, the CSS and CSQ were often in the 50% range. I have also seen some devices connect to T-Mobile (at our factory) and these seem to have CSS and CSQ in the upper 80% range.
Why is it that some devices connect to ATT and some to T-Mobile? I know in the field the devices will probably try to latch onto the strongest signal possible.

I know I can grab the CSS and CSQ via the vitals and I have recorded a handful and plan to monitor this over time.
Does anyone have any suggestions or know if the device has the ability to capture the CSS and CSQ on the device and log it? I thought it would be interesting to have the units log this info over a period of time and then dump it to see how they are performing.

With some of the units deployed in the field, I can see that the CSS and CSQ for a handful of devices the values are around 80 or higher. Most other devices are between 30 and 70%. Some are down in the low 20%.
For most part a lot of the devices currently in the field are BRN402 units running DeviceOS 3.0.0. We are now starting to ship BRN404X units on 4.0.2.
The question I am getting from the customer is, is there any room to improve the RF performance? I am no RF expert so I am rely on just how the devices operate. For all the testing I have performed, the devices seem to just work. Now that devices are being scattered all over, the units show lower CSS and CSQ has the customer concerned.
If we are seeing units that have CSS and CSQ in the upper 80 range does this mean, the design is mostly OK? There are so many things that can influence the RF performance that I am not sure how to accurately determine the over RF performance.

If anyone has any wisdom or knowledge to share, I would love to learn more.