Explanation of Ratios

In an unsymmetrical alkene, the boron atom adds predominantly to the least substituted carbon atom. For example, 2-methyl-2-butene gives the products shown below.

CH3-C-CH2-CH3 (also has a CH3 single bonded above the C and a BH2 single bonded below the C) 2%

CH3-CH-C-CH3 (also has a CH3 single bonded above the C in the CH group, an H single bonded above the C by itself, and a BH2 group single bonded below the C by itself) 98%

Explain the ratio obtained.