(http://i1207.photobucket.com/albums/bb463/Junnapon/simscr05_zps4c576021.png)
Surely something is amiss here. Note that this applies only to arable farm built by the player (as far as I have seen yet).
Hmm - that does not look right. The default values in manually built industries have been problematic in the past: I will have to look into this. Thank you for the report.
James,
sorry for "stealing" your bug, but I had that code opened up anyway. fabrik_t::update_scaled_pax_demand set the adjusted visitor demand to the adjusted visitor demand adjusted again (doubling visitor demand for every field added), rather than to the base visitor demand adjusted just once.
(I still do not understand the necessity of adjusting everything to month lengths, which are highly variable and cause rounding errors everywhere, I must confess.)
Pull request at https://github.com/jamespetts/simutrans-experimental/pull/32
Apologies for the delayed response: I had not noticed that I had not responded to this. As might have been noted at the time, I did, in fact, merge this into the way-improvements branch. Thank you for your work on this.