underflow

[MacPhisto]

What is a underflow, i encode a svcd in another day with cce and the muxerlog says:

ultiplexing file c:\video\vida01.mpg
video PTS (5620.00ms) underflow at pack 844 by 6.67ms
video PTS (6220.00ms) underflow at pack 938 by 33.33ms
.......

What is the cause of that?

Thanks