It doesn't actually have less spices as such, it has less chilli and more cream so that it's less hot. Korma which is legitimately Persian and from the indian subcontinent is more mild than Chicken Tikka Masala. Likewise Makhani and Hydrabaddi are Indian dishes with a comparable amount of cream and hotness. Don't get me wrong, it's definitely not a hot curry, but it's not weaker than some of the things coming from India or Pakistan.