shower niche

I had two niches added in shower while renovating it and the tiling looks great everywhere except for round the edges of the niche. Whys it like this and whats best fix?

Hi. I'm in the middle of refurb - and i've asked my builders to add a second niche in the shower. It's 500x150. I've been told it'll take 2 days - as one needs to cut the joist, cut the tiles, etc. Bathroom currently completely stripped down - and current quote inlcudes tiles throughout and the...