I recognize that they're both using synthesizer on their songs, that I think that they're came from the same roots, but nowadays they likely to expand their type.
For example, I define The 1975 (band) is a mix of synth-pop and rock, but Au Revoir, Simone (band) is kinda electro-pop that likely to synth-pop.
So:
Synth Pop really just means any pop music that is oriented around synthesizer sounds. It's a broad term and most synth pop bands could also be labelled with another more specific genre such as dream pop, new wave, new romantic or so on. You could argue that synth pop in the widest sense doesn't culturally 'come from' anywhere apart from the increasing availability of synthesizers from the 70s onwards.
The term 'Electro' on its own most usually refers to the crossover between early electronic pop music such as Kraftwerk, American DJ-oriented hip-hop and disco cultures, and funk music. One example is Herbie Hancock's Rockit. Another is Afrika Bambaataa & the Soulsonic Force's Planet Rock, which features robotic voices and an emphasised harsh electronic sound.
As for "Electro-Pop" - I don't think that term is widely understood to mean anything very specific. You might notice that Au Revoir, Simone aren't labelled as 'Electro-Pop' in their Wikipedia article, while referenced artists in https://en.wikipedia.org/wiki/Electropop include Britney Spears, LMFAO, and Taio Cruz. It could be just a synonym for 'synth-pop', though it's somewhat redundant these days to say that, as electronic instrumentation is so common. It could also refer to a pop act that references the gritty early 'electro' sound (Such as techno artists do in the 'electroclash' genre).