I have said in threads about not realising being late until after the time that the thing starts, rather than after the time you should have left - usually in the past tense.
But I do also think that you will have a sense of this kind of thing or not. The building in of extra time like whodafeck says. Whether it's learned or innate. It probably is learned, but I have definitely seen people post in confusion when they encounter this kind of thinking and essentially say "But this is so obvious?? How could you not realise?"
Also, it's like many other skills - if you have the kind of brain which processes this specific skill like the majority of people then you will learn how to prepare and leave on time simply by doing it, or observing your parents etc doing it multiple times, probably getting it wrong a few times and then adjusting behaviour/expectations etc. You'll "calibrate" to a normal/fairly accurate extent. Obviously everyone misjudges and is late sometimes, but it won't be constantly. Whereas if you have a deficit in that area you won't pick it up in that way, but only if somebody explicitly sits you down and explains each part of the process to you, and again if you get it wrong, if someone helps you to go back over the process and see where you didn't leave enough time. One of the problems of ADHD and dyspraxia (and perhaps some others, I'm not sure) is not being able to see the patterns of your own behaviour, so whereas someone without a deficit in this area could see "Hmm, this keeps happening and it's always because I forget to factor in parking time." Someone with a deficit will tend to see each event as isolated and simply feel unlucky or frustrated that it's happened again or it's "always happening to them" - they won't automatically register that it's something they have control over. For example it took me a really embarrassingly long time to click that it's best to get the train/bus immediately before the one I would really need, in order to allow for delays or cancellations. Before realising that, I would see the bus or train delay as something which was completely outside of my control. Because actually the running of the bus is, but my ability to predict that and get the one before just in case is not.
I absolutely do the "Don't need to leave until 2.40" thing if I'm not paying attention. If I actively added up all the steps - the 4 min extra that I'd "rounded", the time for traffic, parking, walking around, not to mention actually getting my purse, keys, shoes etc together to leave the house I'll conclude that I need to start getting ready at about 12.30 or something and my immediate reaction will be "That CAN'T be right!! It can't take NEARLY THREE HOURS." I'll have such a strong sense of needing to recalibrate this to something more "reasonable" that it takes quite a lot of effort to force myself to either check and add up every little thing and/or remind myself that my estimates are usually unreasonable so my feeling that it's far too long probably means it's about right. But my default mode is to forget all of the extra things and underestimate. I can't seem to recalibrate that.