With desktop apps we CAN get the OS to draw the actual controls.
Can’t do that with iOS or Web.
The IDE does its best to draws an approximation of what will appear at run time.
It doesn’t execute the the javascript / html / css that the browser does.
So its only an approximation - it’s close - and we try to make it so close you cant tell the difference.
Then the approximation needs to be adjusted, as the IDE used to match what was produced and now it doesn’t (this is all based on Liz’s information above - I haven’t personally verified that there’s not something else to blame).