PhillD gave you a lot of good information.
Some unsolicited purchase offers are just come-ons or “bait and switch” attempts. The specific terms in a purchase offer can also make a big difference. For example, offers that appear to cover your interest in one specific location, or well, may include deeds worded so that they actually cover any mineral interest you own in the whole County. That said, if your family is receiving multiple offers, and from bonafide buyers, there is justification for them that you should be able to figure out.
From what you posed I’m assuming you don’t have a lot of information on the current activity in the areas where your mineral interests are located. Before anyone here could attempt to answer your specific question on the reasons behind “high” purchase offers versus low current royalty income, they would need specifics on the locations (State, County, and abstract or legal description), the well names and operators you are receiving those small monthly royalties from, and whether that production is holding all of your interest, or if not, whether that mineral interest is currently under lease but not in production.
If you want to post those specifics I expect someone here will be able to help you.