Some phones have running apps that use a bit of data. We reserve enough credit to cover 2MB for that open data session in cases when you're out of data or credit.
When you topped up, this reserved amount was deducted from your balance if it was used or returned to your balance if the data wasn’t used. As a result, your credit balance may show as less than the amount you topped up. And you may not be able to buy a Plan for the exact amount you just topped up.
For example:
No probs for next time though. If you have a data session open when you run out of credit or only plan to Top Up the exact amount for your Plan, turn off Mobile Data in the Settings menu in your phone before you Top Up. This will make sure there is no data session is in use when you Top-Up and the full amount can be used to purchase your Plan.
If you haven’t got enough credit to renew your Plan, you will go on the Standard Rates after your Plan expires.