为什么我按时还款，但余额中的本金/利息却在波动？

我在2011年3月6日第一次还款时，本金是$261.11，利息是$221.64。随着我不断按时还款，我的本金不断增加，利息不断减少。但在2011年3月11日，我的本金减少了，利息却增加了。2012年3月4日，同样的情况也发生了。

我以为我一直按时还款，每次申请的利息应该减少，申请的本金应该增加。难道我错了吗？

我的贷款还款记录表是这样的。

**04/03/2012 Payment Received $482.75
    Interest $160.96      
    Principal $321.79**        
03/02/2012 Payment Received $482.75
    Interest $142.77      
    Principal $339.98      
02/03/2012 Payment Received $482.75
    Interest $154.97      
    Principal $327.78      
01/04/2012 Payment Received $482.75
    Interest $157.34      
    Principal $325.41      
12/05/2011 Payment Received $482.75
    Interest $169.92      
    Principal $312.83      
**11/03/2011 Payment Received $482.75
    Interest $166.60      
    Principal $316.15**        
10/03/2011 Payment Received $482.75
    Interest $141.54      
    Principal $341.21      
09/07/2011 Payment Received $482.75
    Interest $165.26      
    Principal $317.49      
08/08/2011 Payment Received $482.75
    Interest $172.73      
    Principal $310.02      
07/08/2011 Payment Received $482.75
    Interest $197.05      
    Principal $285.70      
06/03/2011 Payment Received $482.75
    Interest $221.64      
    Principal $261.11