It may be that my brain isn’t fully awake but this don’t make sense to me, if my starts date is on 24th October this year why isn’t this also the start of my 180 day period?
Because you have specified your proposed date of entry as 1/6/20 and the rolling 180 period is being calculated backwards from that date. For each day that you spend in the zone from 1/6/20 the start of your rolling 180 day period will also advance by one day, so any days in the zone at the start date will effectively drop off at the same time as your current stay advances. Days spent in the zone more than 180 days prior to 1/6/20 won't count on your proposed date of entry and days spent in the zone immediately after the start of the initial rolling 180 day period will drop off as your rolling 180 days moves forward.