Because your base period earnings did not qualify you for 26 weeks.
Because $8,700 divided by $425 = 20 weeks.
I assume you are in CA.
Have you read the CA handbook???? You should have received this with your Notice of UI Award.
http://www.edd.ca.gov/pdf_pub_ctr/de1275a.pdf - see pp. 4, 5 and 7.
Handbook clearly describes how your MAXIMUM BENEFIT AMOUNT is calculated.
CA takes total base period earnings and divides that by 50%. That is your MAXIMUM BENEFIT AMOUNT (50% of base period earnings).
Your WEEKLY BENEFIT AMOUNT is determined by high quarter earnings. Your WBA is $425.
Since CA has allotted you $8,700 (which means 50% of total base period earnings of $17,400), you divide your WBA into your Maximum Benefit Amount to arrive at number of weeks.
In short, CA pays 50% of total base period earnings NOT TO EXCEED 26x WBA.
In order for you to receive 26 weeks' benefits, total base period earnings would need to be $22,100 (26x$425x2).