Could be inheritance from another relative such as a grandparent.
Lottery win, not enough to give up work, but enough to enhance lifestyle.
Her partner could earn a lot more than yours.
There could be other things she doesn't spend money on that you do - cars, groceries, mobiles, TV./broadband etc etc . The difference in cost between just paying the normal price and getting a deal is massive.
Half of MN thinks it costs £200 pw+ for basic family groceries. If you shop for deals and eat mostly unprocessed vegetarian food you could do it for under half that, so there's £100 pw for other things right there.
Your phone could be a £5 sim only deal and a £200 Android phone replaced every 4 years, so average cost under a tenner a month or it could be a new iPhone on a £60 pm contract, another difference of £50 a month.
Is your car a £500 pm leased gas guzzler or do you buy a 3 YO small engined petrol car that you keep for 10-15 years?
Some people have 'full Sky' or all the subscriptions, others only ever have one or two and get those at a discount (we currently have 2 that are both half price due to Black Friday deals, we won't renew and will swap to something else before they end).
Repeat for all manner of other things and you have many reasons why some people's money goes a lot further than others.
Then you can get deals on travel and experiences, Groupon, hand luggage only Ryanair flights and less expensive accommodation and weekend breaks in Europe don't cost that much.
If I go to London I wait until I can get a cheap train ticket.
I'm always going to exhibitions and attractions like the Yorkshire Sculpture park, but I have an art pass so it's £80 a year for unlimited entry and exhibitions are free or half price so up to about a fiver or so instead of £10-15+
I almost never buy coffee out of the house and my lunch at work is almost always leftovers. I do eat out fairly regularly but that's usually one course and beer (because that's mostly what I want) rather than 2/3 courses and wine, which would cost far more.
I get loads of points from a credit card and discounted vouchers so my money goes a bit further. I actually make money from credit cards by borrowing at 0% and earning interest on savings. It all adds up and what appears to be a similar income goes much further, it's look after the pennies and the pounds look after themselves in action.