Look at the angle of the guy's back, that's the key to why they say that. For performance, the back will be in a more horizontal position.
You need to be careful, though, how much faith you put into this sticker - if at all possible you should try both sizes and judge for yourself. Also, don't forget that different frames, with different geometries and tube angles, will also come into play. If you test the two sizes, and you still end up thinking that neither one fits you better than the other, at that point you could apply the smaller=performance/larger=comfort principle.
This advice is especially true for somewhere like Decathlon, which is a large enough store that a shop will probably have both sizes in stock (or could get hold of them if you ask).
I bought a hybrid bike from Decathlon many years ago, they had height versus size charts then, but I remember totally ignoring them and buying the size that they said was for someone a whole 15cm/6" taller than me, just based on how comfortable that particular size felt when I rode it. I'm not saying that's how it would be for you and for this bike, it's just how it was for me - it's quite an individual thing.