Maven version numbers are comprised as follows:
<major version>.<minor version>.<incremental version>-<qualifier>
If all the version numbers are equal, the qualifier is compared alphabetically. "RC1" and "SNAPSHOT" and sorted no differently to "a" and "b". As a result, "SNAPSHOT" is considered newer because it is greater alphabetically. See this page as a reference.
Note that a.b.c-RC1-SNAPSHOT
would be considered older than a.b.c-RC1
.
I'm not sure what to suggest as a solution - this is just how Maven versioning works.