There is a few steps that occur when you scan:
1. it rolls to see if you find a planet based on your scan and how many planets you currently have in your list
2. it filters out all planets that have higher cloaking then your scan
3. it randomly chooses a planet from the thousands that are out there, with the planet's cloak reducing it's chance ratio
4. if it is occupied, it rolls an opposed chance between your scanning and its cloak

The percent value given on the planets tab is only the chance to find a planet in #1, though you still have a chance to fail at #4. This is why you can fail a scan at 99% success.

Pretty much.

good to know... thnx man! very helpful!

if that is true, 100% scanning chance also should have a chance to fail at #4.

And it does fail. The game will show you max 99% chance of finding planets but if you got a lot of scan you will see that the scan won't decrease under 99% for a while. When I started playing and I was researching scanners like crazy I had scan sprees where I was starting at 99% and stopping at 99% because I would find what I needed in 10-15 scans. Not the case now. It's rather frustrating to see that the better your scan gets the slimmer the chances to find good unoccupied planets gets. Kind of contra-intuitive.

its because ur finding all those cloaked enemy planets now. lol

Well instead of enjoying the game at my own pace, it pushes me towards animosity and strife. I shouldn't be forced into it. It should be my choice to make. Plus, you can get your FB friends into playing until rank 20 just to scan and share you planets. So there is a work around, a lame one, but there is. I'd rather have the game mechanics not forcing me to do neither. Oh well...