Just as measuring with cups can yield a variety of results due to the packing of contents, there are differences in types and brands of flour. Not forgetting, flour is milled from wheat, and there are varieties and blends of wheat, and different milling types. Plus, bleached/unbleached, enriched/unenriched... also, if you're using recipes from elsewhere in the world, the flours will likely be a bit different.
Start with checking your flour source on the bag (if you tossed the bag, check in the supermarket or its web site) what the weight per cup should be. King Arthur Flour has some pretty precise weights for all its varieties of flour on its site:
https://www.kingarthurflour.com/learn/ingredient-weight-chart.html
In general, I'm seeing mostly a range between 120 and 130g depending on whether all-purpose or whole wheat or bread flour, so if you're seeing 140, that seems unusual.
Swans Down cake flour (
https://www.swansdown.com/about) says it's 28g per 1/4 cup, or 112g per cup, and that's on the low side. I have been using White Lily, a flour common in the US Southeast, which is made from a softer winter wheat. While I have AP flour in one container, I have White Lily's self-raising flour in another, and it works pretty well in the UK recipes I'm copying off YouTube (Cupcake Jemma's for instance.) I was making my own self-raising by adding the leavening separately, which you can do too. White Lily isn't as light as the Swans Down cake flour, but my cupcakes come out basically the way I want them in Jemma's standard base of 125g each SR flour, butter, castor (or extrafine) sugar and 2 eggs for a dozen cupcakes (plus whatever flavorings.)
BTW some of the tables online contain the line that "King Arthur says all flour types weigh 4 oz (113 g) per cup" - but by their own web site this is demonstrably untrue. I don't know where that came from but some of these sites appear to have copied the info from each other which is why I started looking at the actual flour companies' sites.
In general, the more you can know about the flour the recipe intended (usually they'll state all-purpose, cake, whole wheat, bread, etc, but sometimes in their ingredient notes in a cookbook they may mention what brands they prefer) and the flour you have (from the label), the better. I still find that from recipe to recipe, especially in breads, I may have to add more or less flour. Not forgetting that ingredients like butter (in enriched breads) can also vary in their water/fat content.
I've loved adapting to scales -- far fewer things to wash and guesses as to whether I scooped the flour, or packed the brown sugar, precisely right. I do still use some recipes that require cups, but you can also consider, if you have Microsoft Excel for example, making a set of tables that would convert recipes from cups to grams. I also have one now-standby cake roll recipe (just five ingredients) in Excel so that if I wanted to make it bigger or smaller, I can convert based on the number of whole eggs (before separating.) The original recipe called for five eggs, but the resulting cake is kind of thick for my preference in a roll, so I often make it with four eggs, and each ingredient in grams is easy to calculate out (essentially x 4/5) around the egg count. That would be hairy to do around cups and tablespoons, like when you get those "one cup plus two tablespoons" of flour in some recipes.
Now when I'm searching for recipes online, if I find them metric AND in US-available components, I'm more likely to try that one.
Snapshot I'm attaching is from the White Lily web site, same as would be on the bag with nutritional info below. It shows that the "enriched, self-rising flour" contains ingredients in addition to flour (natch) and the weight per serving size, in this case a quarter-cup, so you can multiply by four to figure out the weight per cup.
