Maternity pay query?

I've used the NHS mat pay calculator that is recommended by many ppl but the calculations seem incorrect so I'm wondering why it's always recommended? Maybe they're right and I'm missing something?? I've inputted my gross monthly income (before deductions) and I'm due to start mat leave at the start of July which should make the calculations easy? So why does my first month show I will get more mat pay than I would get regular pay if I was at work? For context gross per month is 2881, 1st month pay is showing as 2944. Any help would be appreciated 🙂