The firm's clients have political biases. Every poll is biased by the questions it asks - or doesn't ask. Big companies like big pollsters. Got a lot of money - use a huge firm. Got a little money, go to a niche player.
Pollsters compete on reputation and service - clients see distinctions.
Sometimes a company's research specialist defects to a market research firm and takes the specialist's former employer's business. In a general election, a media pollster can seriously damage their reputation by putting out a biased poll. These are the type of surveys where various organizations purchase individual questions and they don't sponsor the entire survey. Party standings from omnibus polls are usually found outside of elections. You can always find something positive to leak if you have asked enough questions.