Different connections on positive and negative pins in PCB design
I'd like to know if there's any good reason behind that


The part is a 470 uF 25 volt electrolytic capacitor like this: -



So, it will have a fairly high ESR (effective series resistance) that will dwarf the resistance of the via to ground and the short copper track. In effect the way that it is mounted will have little effect on its performance.

If it were a non-electrolytic device used in high frequency circuits where the capacitor were expected to deliver anything other than bulk capacitance then how it is mounted with respect to the power rails could be very important.


  Shouldn't both pins have the same impedance path to the power rails?


Ideally yes, but in practice for a bulk electrolytic capacitor, you could put it in series with 0.5 ohms and it would, more than likely in 99% of applications, work perfectly as required by the circuit.