The installs you see in the Developer Dashboard is what you would expect - the number of times a user has added your extension to their browser from the Chrome Web Store.
On the other hand, the number on the Chrome Web Store listing is roughly speaking how many users have your extension installed today. If everyone who has ever installed your extension had kept it, you would expect that to be around 600, and if everyone uninstalled it immediately afterwards, that would be towards 0. Somewhere in between makes sense as in reality some users likely choose to keep your extension and others will try it out and then uninstall it at a later time.