What are the primary differences between Ethiopian and Eritrean food?
I asked my friend who's written a cookbook adapting traditional ethiopian cuisine for vegetarians, here's what he told me. 

Ethiopian and Eritrean food are basically identical. They were the same country until the early 90s, so that's not too surprising, I guess. In both countries, wats are common and are eaten with teff-based injera. Berbere is a primary seasoning in both countries. He said the only differences are due to climate: Eritrea is at a lower elevation, so the food there uses more warm-weather vegetables like tomatoes, eggplants, etc. while Ethiopian cuisine might have more carrots, for example. 

However, he did point out that "Eritrean" restaurants he'd been to in the US (specifically, in the Bay Area) were actually closer to what he would consider Ethiopian food anyway, which may explain your experience: you may be eating Ethiopian food at an Eritrean restaurant. Of course, since the cuisines are so similar, this is like going to a "Western New-Mexico" restaurant in China and being served (gasp) "Eastern New-Mexico" food--not too surprising, in any case.