Well, according to BtN
"Junior is used to distinguish a son with the same name as his father." (no mention of usage for Senior)
Senior is used, I figure, when both
men are being mentioned or when there might be an ambiguity. Such as, on the announcement of your own daughter's engagement. I suppose that it is conventional to have omitted Sr. in this case, since there was no need to mention you on the invite, and the default assumption would be that Robert refers to your dad (while Robert Jr. identifies you). Names are sort of first-come, first-serve that way - he was Robert before you were, so conventionally, his seniority means he gets to be Robert without qualifications.
I don't think you should take it personally at all, especially considering in your small town, folks generally know that there's a Robert Jr as well as a Robert, and that you (I assume) probably have no grandkids of marriageable age.