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The electron beam is turned on for a period of time as it sweeps horizontally from left to right, illuminating parts of a scanline, before it’s turned off again.

Usually, this means that one CRT “pixel” is slightly wider than it’s tall. The height is one scanline, which can be translated into one pixel, but the width can be more than that.
That’s the case for Space Invaders. Because its resolution of 256 x 224 isn’t quite 4:3 like the monitor, the image gets stretched vertically as each pixel occupies a little more of the horizontal scanline.
Note that since the CRT monitor in Space Invaders is rotated, one pixel is in fact slightly taller than wide instead, after rotation. This means that the iconic crab-like space invader looks too squat and flat in many emulators, for example. The enemies should look more or less square.

Since Space Invaders uses an old CRT monitor, the scanlines are actually visible. Because of the phosphorescent glow, they’re not easily discernible, but the gel overlay mutes the bloom on the colored areas, making them stand out more there.