A different treatment is applied when creating the numbers. Spelling numbers themselves is too wordy (numbers like Eight would be hard to make into new forms while maintaining the shape and size of the letters).
The numbers instead use equations to determine their parts. 8 becomes a form made up of two 4s, for example. This allows the numbers to maintain a sense of unity with the letter set. The new forms are created from their counterparts, and this is appropriate as it aligns with the process for creating the letterforms.