Update: Apparently, they clearly did like an estimate without taking into account of other factors.
Instead of 28 days because Hops regrow everyday, they input 27 days.
Although, I'm still hung up on the reason why they do 27 days. Please enlighten me on this.
All of these gold per day calculations need proper review, not just clicking in numbers to an equation and thinking it's a shoe that fits all sizes.
The basic g/day calculation assumes no growth speed modifiers, i.e. no fertilizer and no Agriculturist profession, and uses the base crop price, i.e. 100% of crops harvester are basic quality. It also assumes you do not process the crops, just sell them raw. If your specific farm circumstances means your crop grows faster, and/or you have fertilizer, and/or your farmer's Farming skill level increases the chance of quality crops, you can recalculate for what you will earn, using the tables on the Farming page of the wiki and the crop growth calendars on the wiki page specific to the crop concerned. If you process crops, you can again recalculate
It would be too unwieldy to list every possible calculation for every possible combination on every single crop
And the basic calculation uses 27 instead of 28 as the divisor for single season crops because that's how many growing/regrowing opportunities there are in a season. Growth happens at night, and the season ends, and crops die, on what would be the 28th night, so there is no 28th growth opportunity. You also have to subtract the cost of the seed from the overall earnings
So for hops, it's a basic 11 days to grow, meaning you get 17 harvests. Each base price harvest is 25g earned, and the hops starter costs 60g.
(17 x 25) - 60 = 365g earned
365 / 27 = 13.52 g/day