In theory it's a large range, but in practice it really isn't. Candidates regularly negotiate starting salary to 10-20% above an initial offer, so a range like this is pretty common. You also need to keep in mind that job ads can cost $, so a small company on a budget doesn't want to be placing a bunch of ads for each potential level of hire when all they want is someone who can do the job.

"Junior" and "Senior" probably oversimplifies it. You could have a senior with 10 years and one with 20 years. The salary difference often won't be as pronounced (because salaries tend to plateau at a certain point), but there will usually (and should) be a difference.