Typically the brokerage fees/commision are the same. They get a bit more on the back end based on portfolio performace and volume growth.
The discrepancies are usually do to "assumptions" in coverage. Liability limits, deductables. So, you give the agent your info, age, time licenced, tickets etc. But, one agent may quote you 1 million liability and $1000 deductable while another may go 2 million and $500 deductable.
Keeping in mind the first point, some brokers/agents may discourage some business by quoting the worse possible scenario to try and maintain a clean portfolio. It's not common but, it does occur.
Kind of basic examples but, the principal is the same. The quotes should be the same within the same company as long as all the information they use is consistent.
So, what is the lesson? It does help to call around. Even agents within the same company.
Lastly, the cheapest quote might not be right coverage for you. So, make sure you know what your coverage is to make an informed decision.