The umbrella has a higher rate because the "employment" costs are passed on and paid by you via the umbrella.
I.e. if you're employed directly, it "costs" your client:
Employers NIC
Paid holidays
Statutory sick pay
Enhanced maternity pay (if applicable)
Workplace pension employer contributions
Apprenticeship levy.
If you work via an umbrella. the end client saves all that, hence paying the higher rate. BUT, you bear those costs instead, i.e.
Employers NIC
Workplace pension employers contribution
Holiday pay
Apprenticeship levies
And of course, you pay the umbrella company's admin/processing fee.
And you aren't eligible for the employers enhanced sick/maternity pay (if applicable).
To make a valid comparison, get a quote/illustration from the umbrella company as to how much "net" take home you'll actually receive from them, after their fees, employers NIC, apprentice levies, etc etc.