SJuan's answer is mostly correct, but I'd like to add two points:
Belgium has compulsory voting, which means many more moderate votes will be spread across the spectrum. These are given by people that don't feel very strongly about politics, but still give a vote. Many of these people are completely undecided about what party to vote for (all votes for each election are only valid when cast on a single party/list) and many pick something at random when they're in the voting booth.
Multiple parties with partially overlapping programs combined with a surprising lack of contentious subjects
While most Belgians won't admit it, there are little to no really divisive topics at the level of Flanders or Waloonia compared to other countries. Even at a federal level, some of the larger issues in other countries such as euthanasia, abortions, gun control, religion, LGBT rights, EU support, ideas about climate change and the likes, have quite clear majorities in a specific direction. Even on less clear stances such as social security, the differences across parties are extremely small compared to countries such as the Netherlands/Germany/France. Tensions between the two main regions tend to rise when there's not much else in the news, but while divisive, they're fed by vocal minorities and people realize this.
This lack of contention on major subjects means that votes are decided on smaller issues. This is where parties tend to overlap more, so when a person votes based on one of the few remaining issues, there's always a whole plethora of parties taking a stance across the whole spectrum. This causes even people voting the same way on the same issue to not always end up at the same party.