None of my muffins bake at 400°. That seems pretty excessive.

I just checked my recipes, and my bran, morning-glory, rhubarb, blueberry all bake at 375°

A muffin isn't a fancy cake...muffin tins come in different sizes and materials...baking times vary. Look at them....test with a toothpick, touch the top with your finger to see if it springs back. Use your head and powers of observation to see when they are done....and don't cook longer than they need of they will be dry.
If your oven is hotter, they will cook faster...test your muffins and don't worry about directions.

Agree with Linda...pans and ovens can make a difference. A temp of 350 is pretty standard but some muffin recipes do call for higher heat. I think it allows them to rise faster. Best bet is to start checking a little early with toothpick or spring test.

A lot of what I had to say was similar to the others, above. Caramelization starts at 350F, so that's minimum for toasty. A lot of cake recipes bake at 325F so they won't brown. I, personally, love the browned bits on cakes, but not so much on muffins. Quickbreads, inc. muffins, are a kind of cake. Same principles. The higher the heat, the drier the cake or muffin. If a batter is particularly soggy, where drying it out is preferable, I can see the high heat. There may also be a reaction desired where the greatest amount of steam as fast as possible is wanted.

In general, the quicker the bake time the less important the exact temperature. With a 9” cake, you need the center to cook through without ruining the edges. That isn't a muffin issue, though could become one at high heat. The purpose could be for a drier outside. I don't have different bake times for the same muffins in silicone, steel or Polish stoneware, in papers—I was surprised, but it's true. YMMV. With any baking, however, ovens are wildly variable. Most aren't very accurate, and operate with a wide range around the set number. Received recipes from family, friends and tradition, may have temperature settings that conformed with the ovens used, rather than reflecting the actual oven temperature. Remember the classic instruction to wait 30-60 minutes after the pre-heat light goes out to out in your baking. That was to even out the old ovens and get the whole thing hot, rather than just the sensor. Muffins could be put in when the light for 400F goes out and be done before the oven fully reaches that temperature. But many modern ovens are faster to heat, more thorough and more accurate.

That's the longer version. :) Anyway, if the muffins come out just right, don't sweat it. If they could be improved, adjust a little.