It’s all good. Let me break it down.
Semver specifies versions. These are the x.y.z (plus other optional stuff) triples you see. Nothing should be complicated there.
Tools that use semver to select versions also define syntax for defining which versions are acceptable. npm calls these “ranges”, cargo calls them “version requirements”, I forget what other tools call them. These are what you actually write in your Cargo.toml or equivalent. These are not defined by the semver specification, but instead, by the tools. They are mostly identical across tools, but not always. Anyway, they often use operators to define the ranges (that’s the name I’m going to use in this post because I think it makes the most sense.) So for example, ‘>3.0.0’ means “any version where x >= 3.” “=3.0.0” means “any version where x is 3, y is 0, and z is 0” which 99% of the time means only one version.
When you write “0.9.3” in a Cargo.toml, you’re writing a range, not a version. When you do not specify an operator, Cargo treats that as if you use the ^ operator. So “0.9.3” is equivalent to “^0.9.3” what does ^ do? It means two things, one if x is 0 and one if x is nonzero. Since “^0.9.3” has x of zero, this range means “any version where x is 0, y is 9, and z is >= 3.” Likewise, “0.9” is equivalent to “^0.9.0” which is “any version where x is 0, y is 9, and z is >=0.”
Putting these two together:
0.9.0 satisfies the latter, but not the former
0.9.1 satisfies the latter, but not the former
0.9.2 satisfies the latter, but not the former
0.9.3 satisfies both
Given that 0.9.3 is a version that has been released, if one package depends on “0.9” and another depends on “0.9.3”, version 0.9.3 satisfies both constraints, and so is selected.
If we had “0.8” and “0.7.1”, no version could satisfy both simultaneously, as “y must be 8” and “y must be 7” would conflict. Cargo would give you both versions in this case, whichever y=8 and y=7 versions have the highest z at the time.