Take out the penalties and he's still scored almost double what Casiraghi scored for the club. Casiraghi wasn't really that good, he more or less had two good seasons for Lazio and even in his best one he barely reached a 1 in 2 scoring record. Of course he was better in other areas outside of scoring than Immobile but the difference in contribution is absolutely vast. Casiraghi was that guy that would score 10 goals in his best seasons and make up for his lack of scoring ability by running around a lot. Just because he played in a golden era for Serie A doesn't mean he was actually a very good striker. Guys like Casiraghi and Delvecchio weren't any different from strikers like Iaquinta and Quagliarella imo, maybe even worse.

Immobile has two (about) 40 goal seasons for Lazio now and even in his worst season he scored more than Casiraghi did in his best season. His form for the national team and outside of Serie A doesn't make him less of a great player for Lazio, he's an elite goalscorer when in the sky blue and a major reason they pushed Juventus so hard for most of the season. They've also both won a Coppa Italia so Casiraghi doesn't even have the advantage of silverware.