I grew up in the arcades, as well as working in them much later in life.
Arcade monitors of the early classics used larger dot pitch, meaning, the pixel triads were larger, thus fewer, and thicker shadow-mask. This changes the way these games look as compared to a more modern arcade monitor.
Light tended to bleed past the larger dot pitch monitors much more so that todays fine pitch shadow-masks. The lines of the mask being thicker also created a somewhat more blended image by default as well.
Additionally, I suspect that certain games monitors were intentionally tweaked to be a little de-focused, to make things a little more blended.
Finally, Ive heard some say that the fishbowl effect was not present on arcade monitors. While many modern arcade monitors were adjusted to correct the fishbowl effect... a lot of the classics were shipped without any correction, intentionally.