Zool 2 ECS and AGA are identical except for the colourful scrolling background on the AGA version. They have the same number of levels and main graphics.
The ECS chipset could really only display scrolling games with 32 colours at 50Hz so most games had palettes designed for 32 colours. Other limitations in the ECS chipset are dual playfield mode consisting of 6 bitplanes: 3 bitplanes for the front and 3 for the back giving 8 + 8 = 16 colours. Soccer Kid uses dual playfield mode.
AGA allows 8 bitplanes including 4 bitplanes for each dual playfield so you could have an AGA version of Soccer Kid with 16 + 16 = 32 colours total. The ECS one has a lot of grey in the palette to allow mixing with the limited number of colours.
Basically it would take a lot more work to make a really enhanced AGA version so most games simply cheated a bit.
eg. Seek and Destroy - AGA version gives you a few more graphics on the screen and has a better title picture with a nicer fading effect. To redo all graphics for a bigger palette would take too much work so most people did not bother.