What is the difference between these two sentences: 1- Our shirts are all the same size but they're all different colours. 2- Our shirts are the same size but different colours. * I cannot understand why should one use "all" in the first sentence, while it could be written without it (I know it's an adverb, but I cannot understand its meaning).
For the most part, they are the same but with a minor implication (at least to me)
1- Our shirts are all the same size but they're all different colours.
There are any number of shirts (but most likely 3 or more, it would seem awkward to say it for just two). Each shirt is the same size, but each shirt is a different color. If there are seven different shirts, they are seven different colors.
2- Our shirts are the same size but different colours.
This could either be used for two or several shirts. If it was used for more than a few, (let's say for a choir or a team), I think the implication would be that they are all the same size and a variety of colors, but each color may not be unique.
All is an intensifier here.
In the case of same it doesn't really change the meaning, just emphasises the sameness.
In the case of different, it is still an intensifier, but it potentially changes the meaning a bit; unfortunately in an ambiguous way:
It might attach to "shirts" and mean that each shirt is a different colour from all the rest.
More likely, it attaches to "different colours", and emphasises the range of colours. It certainly implies that there are more than two colours, and probably more than three. But it is not precise, and not necessarily saying that each colour is different from all the rest.