After being to dA, I disagree absolutely. I've seen so much style variations there
I take it this is style differences between different artists?
If so, then what you are seeing is a particular artists individual style. If you look at the MLP artwork these artists produce, you should be able to see that their style for these characters are relatively similar to the last picture they did. There may be a few changes here an there, often improvements as their technical skill set improves. But baring that the MLP characters an individual artist draws/paints should look fairly similar to one another; owing to their own style.
Take Tsitra360 again, they are an awesome artist; but they draw their pony pictures with the same style. Stormblaze-pegasus, another brilliant pony artist; they draw ponies in a completely different way to the show. Their style for the ponies they do draw remains the same through each and every picture they do. There is also NCMares, another stunning pony artist. They do a lot of show accurate style pieces, but occasionally they will mix it up with their own style. Then there is HoneyBee, another great pony artist; their style is nothing like the show at all.
These artists all show consistency in what they draw however, and if they was to make their images without cutiemarks and remove the mane and tails, as well as the colour from the image; they will look like the exact same pony in different poses across the pictures they draw.
I still have a hard time believing that these are a fanart for the very same pony cartoon.
Could you expand on your meaning with this one, not exactly sure on what you are attempting to get at.
If you mean some people, such as the previously mentioned HoneyBee, draw in a style that is far removed from the show? Then that is because it does not fit in with their personal style, and they are wanting to be as unique as possible.
Yes, there also are people that try to imitate the show's style up to 100%, but I'm highly not fond of such fanart.
When it comes to show accurate drawings/paintings, this can be through necessity, such as for practicing. This is because it is always good practice to draw what you see when learning to draw, otherwise details can (and often will) be lost in the drawing process. It is a way of learning and improving on your technical skill set, and can teach you a great deal about drawing. It is how a good art teacher should teach people how to draw in schools, it is how my art teacher taught me and my class when I was at school. It is also how my mum was taught in art collage. It is all about looking for the shapes that make up whatever it is you want to draw. Regardless of if it is a bowl of fruit, a vase of sunflowers, a person, or even a horse. A good art teacher will teach you how to break everything down into simple shapes, even long flowing locks of hair.
Then you have the people do it because they want to stay accurate to the show, some do it because they like the look, some because they do commissions for people who want their OC's drawn in a show accurate style.