Reason for 1/2 watt resistors marked as 2 watt resistors

You can see the white casing as a heat sink that provides better heat transport to the surrounding air.
That is possible because it has much higher surface area than the original resistor (from the picture I guess at least factor 5) and it is made of a material with high thermal conductivity.

So that really increases the wattage of the resistor.

This is possibly a metal film resistor encased in a ceramic case, filled with high alumina cement. For higher wattage, wire wound resistors are used in a similar but larger casing. This type of construction is common as you can see here, here and here.

The ceramic casing increases the surface area as well as the thermal mass. The surface area give higher wattage since it can dissipate more power. The increased thermal mass gives it a higher surge rating.

From the pictures, it seems that the surface area has increased by much more than 4 times, making it possible to be marked as a 2W resistor.

Here's an image from the Xicon datasheet (2nd link):

And one from the Uchi datasheet (3rd link):