That might be the rationale to pick 20euro but that does not explain why they felt they had to lower prices (and they also lowered the top tier plan), which would be the most interesting IMHO.
In addition, we allowed Basic customers to also invoice by creditcard or subscription, which was the main feature requested for Professional. Since isn't not exclusive to Professional, that does lower the total value of that tier.
If you disagree with logic, please let me know :)