Why do you think that is wrong?
Virgin Mobile start airtime plan charges as soon as your SIM is sent out, and then usually charge for one month in advance e.g. my bill was produced on the 15th March and is for the period 15th March to 15th April.
So if you signed up on 27th Feb and only just got your first bill then that looks like it's almost one month in arrears as one off to bring your billing up to date for your airtime plan since you joined, and then the normal month in advance. Future bills should just then be one month in advance.
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I work for Virgin Media - but all opinions posted here are my own