Because the housing and the bolt are different materials, they expand at different rates. In this case the aluminum housing will grow more (2X more) than the bolt due to the difference in their coefficients of thermal expansion.
The reason why this doesn't insta work is we typically heat the bolt, which then conducts heat to the other bigger part, which acts as a heat sink. So its really tough to get both components to the same temperature.
The reason why it works at all or works steel in steel is that the growth from the heat causes some tiny displacement between the parts. Even if the bolt gets bigger in the hole, the relative displacement tends to break loose whatever corrosion has glued the mating parts together. So I think in practice people find it doesn't matter too much which part you heat.
