As far as I can tell based on restaurants in the US, the cuisines of Ethiopia and Eritrea are quite similar. Are there any characteristic differences between the two, or dishes that belong strongly to one and not the other?
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.
Lots of differences.... the spice and the way of cooking... and Ethiopians almost have no tomatoes... actually they don't use tomatos - it is mostly onion based. While Eritrean food has tomatoes and onions and we don't use that much butter and that spicey butter. Actually, Eritreans mostly use the butter when making meat dishes.. otherwise we just use oil.
What we have in common is the inerjera based food and sister sauces that are made differently .... specially the lentil sauce, the chicken sauce, the classic meat sauce, and allicha (cabbige sauce)... Other sauces the Ethios have that we don't have, and we have that they don't have. F.eg we never put meat in vegetables... like never.. they do. WE have hilbet (the best Eritrean food in my opinion), they don't have that.
Though similar on the outside, it is actually quite different experience eating Ethiopian vs Eritrean. The Ethios might miss their food among Eritreans, and the Eritreans miss our food among Ethios. They taste very very different...
We were the same country only in the 30 years by American force... I'll tell you one thing, it never affected the food.