I guess the footnotes allude to the fact that the extant manuscripts have some minor discrepancies between them; we don't have any of the original manuscripts left, unless you include the portion of Mark found in a Cambridge library a few years ago which hs been dated to the AD 50s.
I may be wrong but my understanding is that the KJV used the textus receptus, whereas the NIV used earlier manuscripts? The idea seems to be that the earlier and closer to the original, the less likely it is that there will be textual variants and scribal errors. For all the minor differences between manuscripts, I've never see any proof that major ideas are impacted. I still hold that the Bible in its original form is inerrant and infallible.
Of course if God suddenly told me 'I want you to only read the KJV etc.,' I'd grumble but say 'Okay - but you're going to have to help me understand it.' I'm used to earlier English and read Chaucer for pleasure but I do struggle with the KJV and don't find it that attractive. But that's just my personal opinion.
Jules