These balancing methods are based on the about of time that measurements are taken and how often the metrics obtained from these measurements are used to determine how to route a new session.
With 'Live Balancing' the metrics used by the balancing algorithm are collected over a short period of time so that sessions are balanced more often between links. This may result in one link receiving more overall bandwidth then another but the sessions are balanced more evenly based on the interface weighting.
With 'Accrued Balancing' the metrics used by the balancing algorithm are collected over a longer period of time, thus the amount of bandwidth per link becomes a higher weighted factor. This generally results in the WAN reporting graphs showing bandwidth being more evenly distributed over time.
Both methods are effective, the Live method is the default.