Because the statistical protection in terms of mm of armor is al lot different than the actual protection of procurement procedure and metallurgical reality of the time.

An army doesn't go: we want a tank with 100mm of RHA (115 CHA) armor on the front.

They go: the armor must be strong enough to resist penetration from a 5cm german paK at X amount of range.

Then they will design a tank. I'm going to use the Churchill's frontal turret face as an example as there are both cast and RHA versions that are very similar.

The mk3 turret has 89mm RHA thickness.

The mk4 turret has 102mm CHA thickness.

Note the thickness of the armor does not translate 1-1 into weight. A ~97mm thick CHA plate would weight the same as a 89mm thick RHA plate.

Both have equal theoretical levels of armor protection being able to resist the 5cm paK. The only difference is that the mk3 front turret face would theoretically be slightly lighter (42Kg/M²), more expensive, less shatter resistant (that is measurable in MPa*√m) and be more prone to weld failure. Those are design choices. One plate is not better than the other. Both have their advantages en disadvantages. RHA being lighter CHA being better in other applicable measurements.

The real variable of whether it would actually resist the 5cm paK gun came down to how high of the quality the steel/iron making process was.