Branched chain molecules have lower boiling point than straight chain molecules because branched chain molecules have a smaller surface area where they can be attracted by other molecules. Then, why Isodecane (a branched chain molecule) has a higher boiling point than Decane (a straight chain molecule)?
If you mean 2-methylnonane with isodecane, your data is wrong.
2-methylnonane has a boiling point around 166 °C, which would support the rule of thumb that you mentioned.
The only problem here is the units - the boiling point of isodecane is given in Fahrenheit and the boiling point of decane in Celsius.