Guatelitzi Ablation generate incorrct power charges amount

Guatelitzi Ablation support gem description says: while offering spike persist, they grant you a power charge every 4 seconds. Base duration of offering is 6 seconds. So i put prolonged duration 2 (+35%) suppot gem into offering and now duration should be 8+ seconds. So i should receive 2 power charges from a single spike. But i still receive only 1 charge. Further, i went for Preservation cluster to get more skill duration, but spike generates only 1 charge still. Please clarify the description, or fix the bug.