I can't speak to manually added activities, but it's likely that they won't give achievements for that because then anyone could enter whatever they wanted if they're more interested in getting achievements than actually earning them.

Strava made a change a couple of months ago that has really, really, really pissed a lot of people off. To whit: they now award a PR (personal record) for the first ride you do on a given segment. It's logical in the sense that your only attempt on that segment is by default your personal record on it. But it's terrible in the sense that every time you include any new routes on your ride and hit new segments for the first time you get flooded with essentially meaningless PR achievements. There's a thread on this in the Strava support forums that's now hit like 5 pages of people who hate this, and almost no response from Strava.

If you keep riding new routes you will keep getting achievements for every segment you ride for the first time. If your brother keeps hitting the same segments he usually rides he'll only get achievements when he actually does better than his previous attempts.

If you live in an area where users have defined lots of segments then your 30-mile ride might include, say, 30 or 50 different segments, while your brother's 30-mile ride might only cover a few defined segments. In a city you're typically flooded with segments. Group rides I go on in more urban areas near me will typically have segments between each and every intersection, for example, plus some segments that cover multiple stretches across intersections. The rides will have massive lists of segments ridden. Meanwhile for my more rural rides there may be only few segments. If I get on a long 10-mile rode with no major landmarks on it there may only be one segment (or no segment) defined for that whole stretch. Go ride in Phoenix, though, and you might see 15 or 20 segments for a 10-mile route.

Segments are often defined by people for things that have some sort of natural meaning. For instance, you might get a segment that starts at the bottom of a hill, and ends at the top. Or you have a 4-way stop on a rural road, and the next 4-way stop is 5 miles away, so someone will define a segment for that 5-mile stretch of road between those. There's typically some rhyme or reason for why people are interested in measuring some given stretch of road as a segment.