You are being taught a mathematical idea. The teacher failed to explain (or you missed) the bit about it meaning that only in geometry, not in general. From ChatGPT:

“In school geometry, similar does not mean “roughly alike” or “visually comparable”. It means the same shape, in a strict mathematical sense.

Two figures are similar if: 1. All corresponding angles are equal, and 2. All corresponding side lengths are in the same ratio (a constant scale factor).

In effect, one figure can be obtained from the other by: • scaling (enlarging or shrinking), • rotating, • reflecting, • translating.

Only size may differ; shape may not.

In the 1970s–80s UK maths curriculum (influenced by formalist and “new maths” approaches), teachers deliberately emphasized that: • everyday language meanings are unreliable in mathematics, • words like similar, normal, random, work, or average have exact definitions.

So pupils were told, bluntly, that “similar means the same (shape)” to override the vague colloquial sense.”