The farm has living plants. If it has none, the calculation says so: «No living plants on the farm - nothing to count».
The variety has its days to harvest set, and the plant has the date of placing in a box. Without either of the two there is nothing to count a particular plant from.
This is an ESTIMATE, NOT A FORECAST, and the product says so right in the caption under the result. It counts by the variety term, measured from the date the plant was placed in a box. The real speed of growth depends on conditions the system does not know yet.
The three groups mean different things. «Certainly ready» - by your date the whole declared term of the variety has passed. «Possibly ready» - the date fell inside the variety's range: the minimum term has passed, the maximum has not. «Still growing» - the minimum term has not passed by that date.
Plants that could not be counted DO NOT DISAPPEAR and do not dissolve into the other numbers: they are shown in a separate line with the reason - the variety has no days to harvest, or the plant has no date of placing in a box. The reason is named so that it can be fixed: fill the term in the variety catalogue.
Only living plants take part. Harvested and discarded ones are not counted - they are already out of the bed.
Any date may be set, a past one included: the calculation shows what would have been ready by that day under the same rules.