That is one of my suspicions. Although, if kelp were plantable it still wouldn't be enough for the 100%. If my calculation is right I'd be at 99% then and still miss one plant.
But it's okay now. During those tests I realised that almost all of these stat entries are completely messed up. The collection journal is incorrect in its entries, some other skill challenges are not fulfillable or so tedious that I finally abandoned my idea of having at least once a Sim that finished everything in that respect that can be done.