In addition to what has already been said, DOSBox has a strong focus on games and quite often they decide to not fix something unless it directly impacts a game. Many people were not happy with this, because it would mean that trivial (and also not so trivial) fixes to make some demos, trackers or business software run were never applied. In particular they refuse any patches for the latter (probably scared of possible liablity).
Edit: Vanilla DOSBox is also unlikely to add support for more hardware (e.g. MMX or early 3D accelerators) because they don't want to maintain it and it won't be needed by most games.
In my head I am thinking one of these dosbox branches is also pulling it up to C++17 or something like that, and the main team has no interest in that really. The eXo project has been using about 6 different branches of dosbox depending on the game for particular features. Mostly they use the main branch but for some bits the game just does not work in all modes. Now that PCem has VHD support I could see them ditching dosbox all together (unlikely). But they will probably not use dosbox for 9x games due to size.
A while back I had to compile my own version of DOSBox because the official version doesn't call glClear() when swapping buffers. The issue tracker had a multi-year-old bug report about it, but the developers insisted that the driver was bad despite the OpenGL specifications stating quite clearly that one cannot assume a buffer hasn't been modified when using SwapBuffers and must clear it.
So it really comes as no surprise to me that there are actually many DOSBox forks.
I confirm that DosBox-X runs Imphobia disk magazine which is doing some very accurate vsync timing (basically switching color palettes several times during the screen refresh to display 48 colours on a 16 colours EGA display)
DOSBox-X is a cross-platform DOS emulator based on the DOSBox project (www.dosbox.com).
Like DOSBox, it emulates a PC necessary for running many MS-DOS games and applications that simply cannot be run on modern PCs and operating systems. However, while the main focus of DOSBox is for running DOS games, DOSBox-X goes much further than this. Started as a fork of the DOSBox project, it retains compatibility with the wide base of DOS games and DOS gaming DOSBox was designed for. But it is also a platform for running DOS applications, including emulating the environments to run Windows 3.x, 9x and ME and software written for those versions of Windows.
Although I already see that it has a ton of features on top of the features of DOSBox, and their scopes differ quite a bit.