Why does the STEEM Pending Payout fluctuate (up and down) ?

Hi fellow Steemers
I'm just a few days into this amazing venture and have the sort of newbie question which I guess many of you have seen or answered before. I would usually ask my son who is well into this and introduced me onto here - however I can't contact him for the moment so I thought it would be timely to reach out into the STEEMIT community for a solution.

So here goes - whilst watching the voting and pending payment amounts in my introductory post (some would say exciting and others would say sad) I noticed the obvious movements as people upvoted but also spotted that there were variations in this amount at points where there had been no additional votes cast.

In general, these were small amounts of just a few cents. However, the most significant change was of around a 20% reduction in the payment amount overnight in the final hours before payment.

So my questions and queries are -

• Is this normal?
• Is it down to variations in influence of the upvoters
• Is it due to market factors within STEEMIT
• Surely there's not a hidden tax !

Or is it something else altogether?

I'd love to hear your take on this, so just let me know.

Thanks

Tony