Questions regarding odd measure numbering in musical music

[BlueNote]

My high school is putting on the spring musical this May, Crazy For You. This is my first time playing in a pit orchestra (actually, in this case, a stage orchestra because we don't have a pit). The production is filled with Gershwin standards, like Embraceable You, I Got Rhythm, etc.

There are 24 songs in total. A HUGE stack of music.

Anyways, there are a couple of things that I noticed almost immediately when we started rehearing through the music.

1. On a majority (if not all) of the songs, there are usually parts of the song where it has...erm, how can I put this... multiple measures of the same measure. For example, a normal piece of music would show the measure numbers 1-2-3-4-5, etc. In this case, some parts of the song have numbers like 10-a 10-b, 10-c, etc. It's one measure but are visually separate measures. Why would they do this?

2. On one song in particular, at one point in the song the measures went from 1-120 and then to 50-something, then BACK to 120 later in the song, then BACK to an uncronilogical number. Why in the world would they do that? It just makes things more confusing when rehearsing.

[Merlin]

Anyways, there are a couple of things that I noticed almost immediately when we started rehearing through the music.

1. On a majority (if not all) of the songs, there are usually parts of the song where it has...erm, how can I put this... multiple measures of the same measure. For example, a normal piece of music would show the measure numbers 1-2-3-4-5, etc. In this case, some parts of the song have numbers like 10-a 10-b, 10-c, etc. It's one measure but are visually separate measures. Why would they do this?

2. On one song in particular, at one point in the song the measures went from 1-120 and then to 50-something, then BACK to 120 later in the song, then BACK to an uncronilogical number. Why in the world would they do that? It just makes things more confusing when rehearsing.

The discontinuity in measure numbering results from the arrangements being revised in rehearsals. Sections are added or removed, but the previous bar numbers are retained. In the instance of insertions, letters are used, hence the 10a,10b,10c etc...