I also noticed the same - but unfortunately the Customer Care 'chicks' or 'guys' may not have the true answer...
It goes like this - after 30 days, you lose what you have, unless you purchase another bundle..
What I do, i have come up with the best optimum bundle rate -- Ksh. 2200 for Ksh. 1.1 per MB.
I can buy upto Ksh. 6000 worth of airtime and load them at the self service bundle website - BUT - I will not purchase bundles !!!
Once my 30 days expires, it will go back to 0 -- so I look at the balance of MB left at the end of the month..
1) If its too little MB left, I purchase another Ksh. 2200 bundle from my Ksh. 6000 deposit
2) if there is a lot MB left, I just purchase for Ksh. 200 to refresh my old bundle.
I hope this is the problem you are facing..
w/r