SciFi is fiction about things that are not possible technologically today. Speculative fiction explores theoretical, philosophical, and/or possible states of existence.

For example, fiction about what may have happened if some event in history had unfolded differently. Like, what if Hitler had won? Or what if Hillary Clinton had won? Speculative fiction projects an existence from that what-if event.

Here's a current real example: The Marvel/Disney series Loki. It is science fiction and fantasy, but it is also speculative fiction, exploring multiple what-ifs in its storylines.