Those 3_X versions refer to the version of bioconductor that they were release for.
For instance, the current version of bioconductor is 3.10, which you can see from the News section of the home page, or up top on the Install page.
When you navigate to the package home page for PureCN, you'll see that you can see "release" in the URL there, which corresponds to the version release of Bioconductor (3.10) [it even says so under the description of the package). This corresponds to a version 1.16.0 of PureCN (look in the Details section, there is a "Version" row).
(FYI: you can hack the URL to see what version of PureCN was using in the previous version of Bioconductor by changing "release" to 3.9, like so: http://bioconductor.org/packages/3.9/bioc/html/PureCN.html)
The fact that you have v1.17.1 installed suggests that you somehow installed the development version of this package.
If you're not sure how you got here suggests that perhaps a few things might be haywire with your installation. Maybe you can start by running
BiocManager::valid() to see if it gives you any helpful output. If you're having problems parsing that, perhaps you can update your question by copy/pasting its results in the question itself.
Also, if you want to understand how versioning works in the bioconductor universe, you can read through the Version Numbering page which goes through the scheme in more detail.