As far as I know there are two differences.
a) Google claims ownership of things you produced off-hours with no company resources if the work you were doing off-hours is your job. If you are for example a DL theorist, you are being paid in part for your general skill at coming up with DL theories, not just for the hours you put in at the office. If you come up with a new DL theory in the shower instead of at your desk, Google still owns it.
b) Google claims ownership of innovations made by employees off-hours with no company resources if the innovations in question were already being researched elsewhere at Google or the innovation was already known to Google. Like any sufficiently large technology company, Google has made patentable discoveries that it has elected not to patent so as to maintain them as internal trade secrets. This is to avoid the mess that would ensue if an employee independently came up with one of these discoveries and filed a patent for it and Google had to dispute the patent by demonstrating prior art and in the process revealing their trade secret.
So if you have a new innovation that was produced off-hours and without company resources, you first submit it to Google's legal office and they will let you know whether they (a) consider it part of your job or (b) were already aware of it, and if so they will claim ownership. If you disagree with their decision there's a third party arbitration process so that you can hash it out without risking the exposure of a trade secret.