While in manual learn, battery gets discharged down up to 0%. iBBU07 & 08 are exactly same in that respect, during manual relearn. For iBBU07, the Cache Policy stays in WT(write through) all through the discharge phase. But for iBBU08, the cache Policy switches to WT, only for the duration that the battery remaining capacity is less than 29% of the Full charge capacity(this is also known as capacity Threshold for write back).
While being in Auto learn, as the battery gets discharged only down by 37%, the remaining capacity ends up staying above 29% of full charge capacity (capacity Threshold for write back) that I mentioned before, and hence the cache Policy never switches.