Yes that's possible because of #6196: a brand-new empty primary might be allocated on a node above the low watermark. The reason for this is that you might pass the low watermark on all nodes without realising it. If new primary allocation were blocked and you're using daily indices you might get to midnight and create the new day's indices, and your cluster health would immediately go red with all new indexing failing. By allowing brand-new empty primaries to be allocated above the watermark the cluster health goes yellow and continues to accept new data which gives you a chance to deal with the low disk space before it becomes catastrophic.
I see. So let me see if I understand the rules (for ES 5.x):
When low watermark is reached on a node, shards will not be relocated to it. But new empty primary shards can still be allocated to it.
When high watermark is reached on a node, shards will be attempted to move from it to other nodes, and also - new primary shards will not be allocated on it. (Meaning for example if all nodes in the cluster are above high watermark, creating a new index will make the cluster health red).