In my experience, "month" usually means calendar month, I.e. a given date this month to the day before that date next month, unless you are paid on a four-week, thirteen times a year cycle. Whereas if they mean four weeks, they usually say four weeks. The problem is that even dictionaries give multiple definitions. including both of those, and I'm not aware of any legislation that stipulates the definition in this context.
Ultimately it's a contractual matter so it'll be whatever one side or other can convince a court it is, but the monetary difference is so small it's not worth taking it to court. So it'll end up being whatever the company's HR say it is. So, contact them, point out the disparity and ask for clarification.
Or, contact someone like Citizen's Advice or maybe ACAS and ask their opinion. Or if really dstermined to make an issue of it, a specialist in employment law. You may find one that'll give you 5 minutes on the phone without charge, and five minutes should be enough to answer this.