Yes, it's kind of misleading the way the OT is shown on the schematic. Usually, schematics show the wire colors associated with the various impedance windings which I find more important than how the symbol is drawn.
I was going by the formula for output transformer turns ratios and impedance ratios. The turns ratio is the square root of the impedance ratio.
So if the primary impedance is 8K and the secondary is 16 ohms, that's a ratio of 8000/16 or 500:1. The turns ratio will be the square root of that which is about 22:1. Say the primary has 2000 turns. That would mean the 16 ohm winding has 2000/22 or about 90 turns. These probably aren't the actual numbers of turns in the TC-15's OT. This is just to illustrate how the formulas work.
At 8 ohms the primary impedance is still 8K so now the impedance ratio is 8000/8 or 1000:1. The turns ratio is the square root of that which is about 32:1. The primary still has 2000 turns so the secondary will need about 63 turns.
At 4 ohms the primary impedance is still 8K so now the impedance ratio is 8000/4 or 2000:1. The turns ratio is the square root of that which is about 45:1. The primary still has 2000 turns so the secondary will need about 45 turns.
Hope this helps explain things. Here's a website that has a calculator for all this.
http://www.maxmcarter.com/classecalcs/tratiocalc.html