Let me introduce you to Jeff (photo kindly provided by Google Images). Jeff is an incredibly smart guy. He solves really all the really hard problems with very little code. These days he works for Microsoft as that’s where the hard problems are today. But back in the days he worked on Flash. And that was when Flash had the problem that startup was slow for large files. So Jeff came up with the smart solution to just limit the initial read to 64kB, or 65536 bytes. So files that are 64kB or smaller will be completely parsed when the first frame executes, larger files aren’t.
Does that matter? Yes, because there’s Flash files that assume that they’re not loaded completely in frame 1 and otherwise hang in an infinite loop (I’m debugging such a file right now). So it’s actually quite important to get right. But I don’t think any standard states this as required behavior. So much for usefulness of standards.