Why my solution is deployed in all WebApplication?

Hello,

I have developed a site definition solution with web and site scope features for SharePoint 2010 with last CU. I want it to be deployed in a specific web app.

After deploying the wsp using Add-SPSolution -LiteralPath ----------- SharePoint Powershell command, I deployed the solution in a specific web app with the central administration. SharePoint behaved differently than I was expected because it made my site definition available for all my web apps whereas central administration shows a solution deployed on one  web app. I also tried the deployment with SharePoint Powershell commandline:

Install-SPSolution-Identitymy.wsp -WebApplication http://mywebapp -GACDeployment

Same result. 

The problem is that I really don't want to have it in all my web apps. I should be wrong somewhere in my SharePoint Configuration or Solution. I don't know on what to focus the debugging.

Can somebody help me?

Thanks a lot