firmware 2.15 http://www.n4g.com/ps3/News... maybe sony was trying to cover that up if it was called 2.16 then people will automatically think about 2.15 so they thought 2.17 was random enough and stayed in the teen

actually there is nothing weird with the numbers. The developers do not stop making version only cause they do not release them to the public. Sony stated last year that they will try to reduce the number of forced updates to whats necessary cause some people complained about the regular updates (no idea why someone would do that, but idiots come in all kind of flavors it seems). I wouldn't be surprised if the internal version is at 2.18 already. As 2.17 naturally went through the whole QA system before release.

As usual Sony uses a pretty normal x.x.x Version numbering scheme

M.F.B -> Mayor.Feature.Bugfix

the second . is nearly always ommited cause its easier to represent the version as a number with only one '.'
2.17 = mayor system release 2.feature release version 1.bugfix for said feature release 7

when the first number changes you can expect a huge change in the system, lotsa new features and changes to the core system workings

when the 2nd number changes you will have new or changed features and when the number behind that changes you have a bugfix or stability release for the features in the version released earlier.

Obviously they did not plan to release 2.17 but afaik someone leaked version 2.15 and that could have led to problems with people having different but valid online versions (remember the online servers will only check if you have AT LEAST the required official version, 2.15 was newer than 2.1 hence it would allow connection to the PSN but could cause problems for other users still using the 2.1 version.
So to prevent any misshap they release bugfix version7 and voila, the PSN requires you to update to that and the leaked 2.15 is no longer valid on the servers.

I expect a 2.2x version for the beginning of april as they said the network would get an overhaul by then.