Calculate inductance for 3-phase double ckt line
It's mainly due to rounding errors in the written example. For instance, it calculates a value as being 0.243 but if you do the math yourself it is 0.2437. When they calculate 0.221, it would be more accurate to use 0.2214. When they calculate 0.235 based on those previous values it is closer to 0.236. When you follow it all through you get a value of 0.61345 i.e. very close to what you got.

Beware of rounding errors.