Guatemala is a difficult country. They change the design almost every year. Sometimes the change is a bit bigger and then they get subnumbers. Sometimes the change is very minor so they stay within the same subnumber.
And that's your problem. You are comparing different dates. They have the same subnumber but they are different dates and different dates have minor differences.
In the past, the coins of Guatemala had the subnumbers in the year lines. But now they are removed and that's a bad thing. It's no problem for the 25 centavos but it is for the 10 centavos for instance:

As you can see, there are 2 year lines for 1986, without any further explanation. Although there's a difference, one is KM# 277.4, the other one is KM# 277.5:

Obverse: KM#277.5 has incuse lettering on shield while KM#277.4 has lettering in relief on shield.
Reverse: KM#277.4 has plain field around monolith while KM#277.5 has bushy field around monolith.
When there are variants in the same year the subnumbers have to return in the year lines! @Some_Nerd