Because every company sells the same standardized Plan G benefits, comparing the same coverage across several insurers is one of the only ways to find a meaningful price difference. What isn't standardized is how each company prices that coverage. Insurance companies use different rating methods: community-rated plans charge the same premium regardless of age, issue-age-rated plans base your premium on your age when you first enroll and it doesn't increase simply because you get older, and attained-age-rated plans are usually priced lowest at 65 but increase as you age, sometimes substantially over time.
This is the main reason the lowest starting premium isn't always the best long-term value — and why comparing more than one company's Plan G rate, and understanding how that company prices future increases, matters as much as today's quote. It also helps to understand how Medicare Supplement premium increases work and when changing companies may require medical underwriting.