In a high back booster seat, the forces of the child are distributed to the vehicle through the vehicle belt in an impact, as the child is held by the vehicle belt. If a high back booster seat is installed with ISOfix, the ISOfix anchorages help keeping the seat itself in place in an impact but do not have to hold the child's forces.
In contrast, ISOfix installed child car seats with an internal harness (where the child is secured with the seat's own harness, not with the vehicle belt) always have a weight limitation, as the forces of both the child and the seat are distributed to the vehicle through the ISOfix anchorages only.