There's no accident to the highest channel left for TV use in each stage of the auction - it is intentional.
In the initial stage of the auction, 100 MHz was to be created for wireless use, allowing ten 10 MHz blocks of spectrum. Each block consists of 5 MHz in an uplink band and 5 MHz in a downlink band. The 26 MHz of overhead was for guard bands: at least 6 MHz between the remaining TV spectrum and the downlink band (the initial stage required 9 MHz), 3 MHz on each side of channel 37 (608 - 614 MHz), which is being retained for telemetry and radio astronomy use, and an 11 MHz duplex gap between the downlink and uplink bands.
So, for the initial stage, counting backwards from 698 MHz, which is the upper limit of current TV channel 51, uplink band with blocks A - J were 648 - 698 MHz, and the duplex gap was from 637 - 648 MHz. Channel 37 and its guard bands took up 605 - 617 MHz. That left 20 MHz between channel 37 and the duplex gap, so downlink band blocks G - J went there, while blocks A - F went into the next 30 MHz below the lower channel 37 guard band, or from 575 - 605 MHz. 6 MHz necessary for a guard band brings the wireless band down to 569 MHz, so the next lowest TV channel was channel 29, which ends at 566 MHz.
In each successive stage, the number of blocks created for wireless use is reduced by one, with adjustments to the guard band between the remaining TV spectrum and the wireless downlink band. Beginning with stage 4, the wireless band begins above channel 37 and beginning with stage 5, channel 37 and its 3 MHz guard band is no longer a factor. The last stage planned gives 20 MHz of spectrum to wireless use, plus the 11 MHz duplex gap, plus an 11 MHz guard band between the wireless band and the TV band, taking 42 MHz out of the TV spectrum, or seven channels (six if one considers channel 51 to be gone already).
See page 7 of FCC document
https://apps.fcc.gov/edocs_public/attachmatch/FCC-14-191A1.pdf for the spectrum clearing plan for stages 1 - 9.
