HSMP service standart

This is what is written Working in UK site:
"HSMP

Service standard: 50% of applications to be decided within 4 weeks of receipt in WP(UK) and 90% in 13 weeks.

Current performance: During February, 58% of new applications were decided within 4 weeks and 85% within 13 weeks. If we need to make further checks or seek additional information, your application is more likely to fall into the category that can take up to 13 weeks or even longer.

We are also working through a backlog of older cases as quickly as possible. The oldest applications now being processed were made in the week commencing 12 July 2004"
Dear all, please help me to understand this because this contrast to my maths understanding.
If 50% of applications are going to be decided within 4 weeks, so the other 50% have to be decided in 13 weeks- why they are writing the other 90%. I can't get it!!!
The same nonsence in February 58% were decided within 4 weeks and 85% within 13 weeks!!

My understanding is the 50% of the first 4 week is included in 90%(within 13 weeks). They may complete 100% within 26 weeks. Cumulative calculation.

its basically a '2 Tier' measuring system, and you should also look at each week separately. So say in the 1st week of january (call it week A) they recieve 100 applications, they then put those applications in a 'basket' marked Week A and date ' week ending 7 Jan 2005, they then have 2 targets to meet for that particular week, that is in 4 weeks time (week ending 4 Feb) they should done 50% (50applications) of those applications and in 13 weeks (from the date of receipt) weekending 8 April, they should have completed a total of 85% of the applications which are in Basket A. so if they meet they first target then between week 4 and week 13 they only have a target of 35 applications to meet. the other 15 applications this is why you get some guys in Jan still waiting for the letters but someone in March has already got his. The causes for delay wont be that they would have put the other applications aside but because they maybe verifying information from say NARIC or the Bitish Embassies in the countries of the applicants origin