Me - more often late than early. Not something I'm proud of, as I consider myself highly reliable, even "over-responsible" in other ways. I've examined it to death over the years, and notice the main ways it happens:
1) I don't leave house on time due to forgetting something and having to go back (last week), or not being able to find something I need (yesterday), even though I had found this important piece of paper 3 hours earlier to make SURE I had it!
As a Myers-Briggs ENFP (almost an INFP), I'm off the scale on the N - Intuitive. As an N, I live in the world of ideas much moreso than the physical world. It takes great effort to pay attention to my surroundings (including the clock) to get ready and leave with everything I need.
Us "N's" are far more likely to lose keys or forget something we needed. Our brains track ideas, not physical objects. Checklists can help, and after I took the Myers Briggs, I got a clip for my keys and started physically attaching them to my purse. (That alone eliminated 30 minutes a week of key-searching.)
Albert Einstein was also an off-the-scale "N," and I hear stories that getting from point A to point B was a challenge for him. He was solving the mysteries of the universe, but he couldn't manage to get off at the right bus stop. (I'm prone to wrong turns and accidental scenic routes.)
With men, we call it the "absent minded professor." With women, it's just "airhead." But sometimes we're late because we were solving great mysteries, too. (You'll thank us "N's" later; the cure for cancer will come from a scientist who is habitually tardy.)
2) Then there's the "P" part. I'm trying to do too much and squeeze something else in. Either I squeeze something in before I leave, or perhaps en route. ("I'm 8 minutes early, oh, I should be able to stop by the store and get this one thing...")
The trouble is partly too much optimism (I never expect a long line or not being able to find the item) and partly the P thing (Perceiving). I'm wired to be flexible and consider all options rather than being decisive and sticking to my decision (for instance, to just go the direct route and arrive early).
The "N - P" thing is why I can give you so many options as to how someone can be late. My mind is always exploring, but I don't have the concrete focus that would come naturally to an S - J.
3) I live out in the country and I know exactly how long it takes to get to Bellevue, Seattle, etc. But because I'm coming from 30-plus miles away, there are more opportunities for unexpected construction, traffic, or getting stuck on a country road behind a Sunday driver.
Obviously, the solution is to just leave 15 minutes early, but even when I do, well, see #2.
4) I do prioritize, for instance, for clients or one-on-one meetings (rarely late). But it takes effort.
5) Occasionally there's a higher priority for me. For instance, if I got a poor night's sleep and I'm going to a big event, I might decide to get another hour's sleep so that I'll be alert rather than prompt. Self-care trumps looking good. (So what if some jerk calls me a jerk.)
6) Perhaps I'm late because I'm writing an overly-thorough response to someone's article. ;-)
Hope that helps.