Title is fairly self explanatory.
Lucky Raspberry is an exotic chest piece for hunters to enhance arcbolt grenade and recharge energy the larger the chain, meaning the more targets were hit. If maximum targets are hit, the grenade is automatically recharged. In the URL, there is only one target, and one damage number. However I get my grenade back instantly as if I landed a full chain. No down time at all. Just instant refresh.
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As I understand it, that exotic will either recharge your grenade fully or not at all -- hitting more targets means a higher [i]chance[/i] to [i]fully[/i] recharge it (and a full chain grants a 100% chance). Since you hit one target, you had the lowest chance to get that instant refresh. I think it's working as intended and you got lucky there. :)