Explain 1920x1080p

There isn't one; 1080p is a common shorthand for 1920x1080, so if someone put 1920x1080p I can only assume it was a simple clerical error made by someone who does not know that 1080p is shorthand for 1920x1080.

There is at least a possibility that 1920x1080 could refer to 1080i, the i meaning interlaced vs progressive or p, but if I were a betting man, I would put my money on it being a simple clerical error.