Every pokemon has the same shiny chance of 700/65535 (or slightly more than 1%). If you're hunting in a certain area, then the pokemon that more commonly appear normally are also going to be more common for you to find as shinies. i.e poochyena has a 40% encounter chance there, while numel is 15% and grubbin is only 8%. So for example if you did 10,000 encounters, the statistical average you'd expect with 1% shiny rate would look something like
4000 poochyena encounters, 40 of them shiny
1500 numel encounters, 15 of them shiny
800 grubbin encounters, 8 of them shiny.
So while they all have identical shiny chance, their regular encounter rate affects how much you'll see each overall. And even with the expected averages, it still comes down to chance so you get fluctuations, especially in a small sample, like getting a shiny for meowth, which only has 2% encounter rate in the area, before numel/grubbin.