Even exactly the same model phone with sequential serial numbers will have differences in electrical performance. Fact is, some phones may have a slightly more sensitive receive module, others slightly more powerful transmitter sections. So long as the phones fall within the range of specifications set by the manufacturer when tested, they are released for sale.
It is no different to two identical cars having slightly different performance because of mechanical tolerances and state of tune of the engines.
Can you tune up your phone to get better performance? No.
Minor differences in component and antenna positioning within the phone case during the assembly stage account for most of the differences in performance. Sometimes you get lucky and get a phone with everything perfectly aligned, other times one of the antennas is twisted a bit out of shape because something snagged as the case was put together. The phone works fine, just not quite as well as it could if everything was perfect.
The signal bar display is only an approximation of signal strength and generally isn't accurately calibrated. There are hacks available on the Internet for various smart phones that allow you to display a number instead of the signal bars. The number is representative of the signal being received by that particular phone.