Why do some synapses grow stronger during a burst while others grow weaker? · Grey Matter

It depends mostly on how likely the synapse is to release at the first spike.

Why do some synapses grow stronger during a burst while others grow weaker?

It depends mostly on how likely the synapse is to release at the first spike. A synapse with a low release probability keeps most of its ready vesicles, and the calcium left over from each spike adds to the next, so release grows during the burst (facilitation); a slow, high-affinity calcium sensor, synaptotagmin-7, contributes to this at many synapses. A synapse with a high release probability spends its ready vesicles on the first spikes and cannot refill them fast enough, so release shrinks (depression). The same presynaptic train therefore reaches one target as a growing signal and another as a fading one.