I think you are cherry picking trivial examples, where people are literally saying "even stuff as trivial as birthday cards all adds up" to then say, "this is trivial" and then ignoring the many other posts with far more relative content and illustrative examples.
With regards to "what bearing does it have on you" if you don't send the birthday card to MIL, well, she thinks my own son didn't even bother and his wife didn't even remember or remind him. So she thinks worse of you both. If you don't pick up the dry cleaning, then DH goes to his meeting wearing the only suit he has left in the house, an old one, ill fitting and dishevelled, and presents like this in his meeting, the boss takes note, unfavourably. A couple of incidents could be the difference between two very similar candidates getting the promotion and not. If I don't think about the meetings DH has, and meal plan around what times he will be home, and ensure the right clothes are ready, we would rarely eat together, and he would be in a flap every morning. Interestingly he also has ADHD (hence I take almost the whole mental load, he just wouldn't remember things, and not for trying) and I actually tried explaining mental load to him about a month ago, and much like this thread, it was like I was speaking Russian.
I have twins. So the epitome of your example of two being the same age. It's not just double the practical stuff. That's not mental load. That's physical chores. And don't get me wrong, there are indeed two lunches to make, two bathes to take etc. But the mental load side is that boy was ill on his last jabs so didn't have them, so he needs rebooking, so I'll need someone to have her while I do that. And her friend has asked her to her party, so I need a present and outfit and need to be in the next town over for 12, and take him somewhere while we wait, but get back by 2.30 to take eldest DC to train station. So then twins will need tea, but eldest DC won't. I'll need to get the bedding washed when I get back, 4 sets, and even that is making sure I wash in the right order so the twins sets are dry by 6pm and while they're drying, DS's can start being washed. All whilst doing this, DH is hosting at the races for work clients, so he'll be back late and eating late, and won't be around to speak to the vicar about twins baptism, so I'll need to run the hoover round quickly now because once DTwins are in bed I can't. But I'll need to get our tea on now because I can't be cooking when the vicars here. It's having remembered to fill in all the forms for the vicar. And the form for boy twins rugby, plus I know he's grown out of the kit, so I'll need to have a look for that tomorrow...but only between 10 and 11 because MIL is coming for 2pm and I need to pick DH up from the station by then, but not forgetting boy twin will need to nap by then, but girl twin won't, so actually I'll drive to get the rugby uniform because that way he can sleep on the way back in the car and maybe I'll drive the long way back to make sure he's extra rested and not cranky when in laws arrive. But girl twin will be getting restless in the car that long, so remembering get the dvd player ready to take tonight, and charged up, so she's quiet while he sleeps, and then potentially we can drive straight to DH.... And so on, and so on. Everything has a knock on effect, and you suddenly have to re plan the whole day because one link in the chain changes. And when there are 5 links, this is frequently.
Do you see it's not just having two more pairs of shoes to put on, or breaks to make. It's the juggling and meticulous planning to make many things align. And align successfully, not just clusterfuck and bodge our way through life. It's a huge mental load. The physical doing part is the easy bit.