I can't say for certain why your bid was rejected, but I think you are partially right (I saw the spec that the job poster was asking for bids on). I see you live in Poland, so US $900 might actually be a reasonable rate. You should make a point of saying that in future bids, because if you were living in the US that rate would be a joke, and would make you look unprofessional.

More importantly though, your time estimate is ridiculous. There is no way anyone, even a very experienced developer, could do what the poster wanted in 3/4 of a month. It might, theoretically, be possible with a team of very experienced developers, all of whom were used to working with each other. Maybe.

The spec itself was not nearly complete enough to give a good time estimate for it, but it's about a 1.5-4 month job for one reasonably experienced person. Anyone who was way outside of that range (<1 month, >4 months) probably does not have enough experience to be doing these sorts of bids.

FWIW, I responded to the poster and told them I couldn't offer a bid because the spec was not sufficient, but that I'd like to offer my services in helping them write a better spec so they could ask for better bids.