Help me understand the TDP/ Temperature of laptop CPUs please

[Lumia925]

I have an HP Core i7 5500U CPU. Base clock 2.40GHz, Turbo 3.0 GHz (the laptop attains this turbo, there's no cap imposed by HP).
Just purchased a Dell with a Celeron 3215U, clocked at 1.7 GHz.

Both CPUs are 5th gen, with 15W max TDP.
The i7 CPU heats up to 75-80 degrees (gaming with nvidia DISABLED in optimus).
The Celeron, playing the same game, reaches a maximum of 50 degrees.

How's this possible? If both have 15W TDP, shouldn't they heat up to the same temperature while playing the same game at same settings?

Is this a fault of the cooling system in the HP laptop? Or is it normal for the i7 (with same TDP) to heat up more?

 

[Ecky]

TDP = thermal design point. The i7 will not exceed 15w average heat output over a period of time, but may greatly exceed that for short periods - the mass of the heatsink is expected to be able to absorb these bursts. The Celeron will also not exceed 15w. This doesn't mean that both CPUs are producing 15w, just that Intel expects both of them to run in a laptop that can dissipate 15w of heat.