No tree is ever alike. Even when taking cuttings, they sometimes behave totally different; one might have a long root that shot down and provides it with whatever it needs, while the other might take a year extra to gain some ground.
I've even reported spontaneous ploidy changes, in which plants - more or less - spontaneously doubled, or quadripled their chromosome sets. Even though they were specifically bred to be diploid.
In essence it boils down to "it could be a bunch of things". You might have hit a critical vein, one might have a better root system, or maybe one has a faster (or delayed!) damage response.
I'm willing to bet that even Pando has genetic variance if you compare samples from different outer edges of the organism. With two trees from different seeds, the difference can be huge. And so will their response to damage.