Rainbow Spectrum with Blend Tool

Good Day everybody,

maybe a simple question but the internet tutorials do not really help me in that aspect.

I want to create a continuous rainbow spectrum (running red - orange - yellow - green - blue - purple) in a rectangular fashion, just as the ones one can see in science books. I have set up individual rectangles, filled them with the respective colours, aligned them in this order along an auxiliary line and removed their outline. The spacing in between the rectangles as well as the size of the rectangles is identical. Only the two "border" rectangles feature half the width compared to the ones in the middle.

Upon using the blend tool in between the individual rectangles, I manage to get the desired colour blurring from yellow to green as well as from green to blue, i.e. the continuous flow of colour forms a nice band, with the height of the original rectangles.

However, upon taking the same approach for blurring the orange rectangle to the yellow one as well as the red one, the resultant colour flow does not maintain the height of the original rectangles but narrows stepwise in both directions, reaching a minimum in the middle between the respective two rectangles and then, also stepwise, growing back to the original height.

How can I maintain the height of the rectangles in the resultant flow of colours? Or to ask in another way - why does the blend tool behave in two different ways although the same approach is taken?

Thank you in advance!