Under the estates page, I see that I can expect to levy around 25 gold. Being the ruler of the region, all of that gold belongs to me. The other two estates in my region - both being empty - each expect a tax income of ~15 gold, from which I as the lord get half (Lord's Share of 50%). That would leave me with ca. 40 gold (40 + 7,5 + 7,5).
You get the full share of empty estates, but empty estates only produce 50% of the income they'd produce if they were occupied by a vassal. So halving the ~15 gold is probably throwing your estimate off.
Also, the estimates are based off the current regional stats (production, tax rate, population) that day, extrapolated for the week. Every day, gold is collected and added to the vault. On tax day, its distributed. So anything affecting those regional stats will affect the estimate you see that day.
