Do you know how a chronograph works?
It measures the time taken for a projectile to pass through two separated beams of light. Now, in general terms any measurement of velocity calculated from travel time will be more accurate if it's measured over the greatest distance possible. "Proper" shooting chronos are relatively large for this exact reason. The smaller the unit is the better the quality of electronics needs to be to provide sufficient resolution to provide an accurate result. The higher the quality of electronics used the higher the price point tends to be so take for example the average Xcortech chrono. It's sub £100 and it's quite small so firstly you need to take into account that the measurement distance is quite short and that even taking into account the mass buying power and relatively low cost of electronics, the guts are not going to be precision instruments. Add to this the effects that ambient lighting can have on the sensors and you are piling inaccuracy on inaccuracy. On top of all that most places don't know how to spell calibration let alone actually have it carried out on a regular basis.
Now, in your example it's perfectly possible that the chrono your brother normally uses at his regular site reads low and that the sites chrono at your regular site reads high, all within a sensible tolerance on their own, but when you combine the tolerances they result in your brothers "perfectly fine normally" gun being over the site limit. That's the main reason I never try and get my guns right up to the limit allowable on site, I like to know I've got a bit of leeway.
Of course if you ever find yourself at the Malls with their regularly calibrated Skan chrono, if they say your gun is over, then it's over.