Why we use singular form after plural?

Example 1:

Three gangsters, whose names are unknown, robbed a bank.

We talk about three gangsters and we use plural form. Everything is clear here.

Example 2:

Consider the following passage from a hypothetical grammar book, or maybe corporate style guide for in-house writers.

• With simple sentences - use a comma.
• With complex sentences and in cases when the type of sentence is unknown - use semicolon.

Here, only the second part is the subject of my interest:

• With complex sentences and in cases when the type of sentence is unknown - use semicolon.

We say "in cases" (plural) because it's not one, single case. There could be different cases. So we use plural form. Okay.

But then, we say "type of sentence" (singular). Why? Isn't it very illogical? Why we use singular form after plural?

Solution 1:

Try splitting the sentence in half and using the plural in both halves.

With complex sentences and in cases

when the types of sentences are unknown - use semicolons.

Sounds a little strange right?

Despite there being multiple, theoretical cases, in each case, we are concerned with a single sentence in particular. More importantly, the type of said sentence. Therefore, it uses the singular.

Hope my explanation makes a bit of sense.

Maybe these two similar examples will help, one with the singular and one plural

In battles, when the type of weapon is important - choose wisely.

In battles, when the types of weapons are important - choose wisely.

The first sentence insinuates that you have one weapon, the second that you have multiple. Both are grammatically correct, just have slightly different meanings.