Megapixels tell a difference
Apart from obvious lack of experience and technical ability, there may be another reason why beginners usually have lower RPI (return per image): camera file size.
(return per image for who doesn't know yet, is total sales amount divided by the number of images you have on sale. Example: 1000 dollars of total sales, divided by 500 photos on portfolio would make for an RPI of 2)
Let's say I'm a designer and I have a choice between two images similarly good, I would tend to buy the bigger one (it allows for cropping and better playing around with my layout) especially if the price is almost the same (and that's true for all the new images, be them from a beginner or seasoned pro).
I don't want to say that a camera make all the difference, but the possibility to upload a bigger file may increase return per image. Don't forget that is a very competitive world, this our of (micro)stock photography.
A lot of sales are for blogs and the web, true. Don't forget anyway, that the best earnings come from images that sells for printing size, especially L and XL. Plus, a portfolio that shows images in all the sizes up the the XL make a better impression of professionality and give to clients a good choice.
Here beside is the image with biggest size I have in my portfolio (5000 pixel at the largest side) Yes, it is a composite, but having now a Nikon D300 my new images will have the XL size more often.