Unexpected new full backup

I show at the bottom of this note the sequence numbering, dates and filesizes of a current ongoing backup. Can someone explain the mystery? Why was a new full backup made on February 17? Why wasn't the series earlier than that time continued? What happened to the original full backup that occurred sometime prior to Feb 7? and where are backups numbered 2 and 3 (presuming that number 1 was the original full backup) that were part of that series? Are backups prior to Feb 17, when the full backup was made, valid backups as they can no longer reference the original full backup from which they were incremented?

And, I suspect that as part of this mystery, were I able to understand why the series is prefixed by C_2_4, C_2_10, and C_2_11 I might have some clues, but why does it start with 4? And, where are 5 through 9? And, why do the identifier numbers sometimes repeat?

C_2_10_2010.tib 2/10/2010 10:32:47 PM 1,092,707.50
C_2_11_2010.tib 2/11/2010 10:33:55 PM 1,259,085.00
C_2_11_20102.tib 2/13/2010 6:42:10 AM 5,227,208.50
C_2_11_20103.tib 2/13/2010 10:30:51 PM 850,508.50
C_2_11_20104.tib 2/14/2010 10:30:45 PM 997,114.00
C_2_11_20105.tib 2/15/2010 10:32:52 PM 845,751.00
C_2_4_2010.tib 2/17/2010 4:28:50 AM 257,129,172.00
C_2_4_20102.tib 2/17/2010 10:31:59 PM 1,097,664.50
C_2_4_20103.tib 2/18/2010 10:34:25 PM 1,331,377.00
C_2_4_20104.tib 2/19/2010 10:32:04 PM 1,143,505.50
C_2_4_20105.tib 2/20/2010 10:33:38 PM 942,072.00
C_2_7_2010.tib 2/7/2010 11:07:44 PM 2,275,185.50
C_2_8_2010.tib 2/8/2010 10:35:11 PM 3,585,393.50
C_2_9_2010.tib 2/9/2010 10:33:14 PM 969,364.50