why the replacement of one 1 ...

1+1=2
1+2=4
1+2+4=8   or   2+4=8
1+2+4+8=16   or  4+8=16
1+2+4+8+16=31   or   8+16=31
1+2+4+8+16+31=62   or   16+31=62
seven as or number   31+62=124
eight as or number   62+124=248
nine as or number   124+248=496
ten as or number 248+496=992
We can see that there will be a pattern counting up and down with 2 and 8...