system
Given the following post, respond as if you're commenting on Reddit:
human
[Magic The Gathering] What is the flavor justification for being able to summon Planeswalkers from different points in their lives?

For example, you can summon Garruk Wildspeaker, or you can summon Garruk, the Veil-Cursed.  There's also their Magic Origins versions of them from when they first became a Planeswalker.  When you summon a Planeswalker, you're summoning them from across the planes for help, and then they go away when they run out of loyalty to you.  Are you somehow summoning them from across time, too?  If so, why can't you summon more than one of the same Planeswalker?
gpt
The planes are not just other dimensions in space, like the alternate dimensions of science fiction popular in several prominent low-mana planes, but alternate times, alternate rules of physics and magic, alternate *everything*. All that may exist, has existed, will exist can be summoned in a duel; but of course, the duel itself is its own reality, so it may or may not affect others. Or both.  

Of course, the magic user (almost always a Planeswalker when you're talking magic on the level of summoning other Planeswalkers) remains a constant in this reality. While they may grasp, intellectually, the truth that there are multiple Jace's available to summon, controlling such powerful magic *and* reconciling the reality of multiple Legendary incarnations is too much, even for beings such as this.  

Even a multiverse of infinite possibility has its limits.