Install-SPSolution will always install "old" solution

I'm installing a Farm Solution on 2013 the following way

Add-SPSolution path/to/solution.wsp
Install-SPSolution -Identity solution.wsp -GACDeployment -Force
Enable-SPFeature -Url $site.Url -Identity solution_feature

The Solution will be installed globally, and the feature will activate a Timer Job as soon as it's activated.

I installed it the first time, everything was ok. But I needed to change something. -> New Development (VS2013) Publish -> generate new .wsp file -> Deactivate Feature, Uninstall Solution, Remove Solution and reinstall the same way as described.

But even though I war removing the old solution, I did an iis reset and restarted OWS Timer Service, after installing the new wsp, it still kind of installs the old version.

I have the feeling it always installs the same .dll file to the GAC.

What could I do wrong?

The only way I found to get rid of this behaviour is to create a totally new project in VS2013 and generate the .wsp file with the new project.

I can see pretty fast that the old solution is installed, because I renamed the Timer Job, but it still has the old name.