Ruby do/end vs braces
That has to do with the difference in associativity of the { character and the do keyword.
In the first case, the block is interpreted as a block argument to the map function. The result of the map function is the argument to the p function.
In the second case, the block is interpreted as a block argument to the p function, while the a.map is interpreted as the first argument to the p function. Since a.map evaluates to a, this prints the original array. The block is effectively ignored in this case.