I think it was considered better to let units shrink due to losses and work with these smaller units as long as possible - a depleted unit which has strong coherence was better than a larger units which lacked it - then bring them back to authorized strength when you have the opportunity to incorporate the replacements, i.e. training time is available, more a "batch" approach in contrast to the "continuous flow".
IIRC the same approach was used by the British army in the Napoleonic era.
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