In the previous leveling system, level 5 and 15 would give no stat bonus, while the next level would give double
This was changed when leveling was changed. Now, all plants have their stats increased by 1.15x per level in a perfectly even way.
It seems as though sun producers haven't gotten the memo yet, and still follow the old pattern.
Example Sunflower produces 25 sun, and each level increases that by 2. It's stats SHOULD go 25, 27, 29, 31, 33, 35,37,39,41,43. However, from level 4 to 5 sunflower gains no sun stats, still producing 31 sun. Then, at level 5 to 6 its stats increase by 4 instead of 2, which is double.
There is ONE sun plant that was properly updated, which is Suntower. Suntower starts at 130 and gains 12 each time. From level 4 to 5, it properly goes from 166 to 178 sun produced. Every other plant follows the old format.