Why is the 多 in 每个月150个多小时 misplaced?

Question

I have a new teacher, and we've been communicating on WeChat. I wrote this:

我大部分时间在自学。目前我每一天花5个多小时在学习，包括周末（每个月150个多小时），不过……

She says that 每个月150个多小时 is incorrect and it should be 每个月150多个小时. Googling “100多个小时” (2+ million hits 101 hits) and “100个多小时” (16 hits) indicates this is indeed an error of some kind.

Question: Why is the 多 in 每个月150个多小时 misplaced?

I searched multiple questions on this topic: 1. Does 多 always come after the measure word？ 2. How to correctly use "多" to say "more than"? 3. Can you still use "Number + MW + 多" when the Number can be more than one MW larger? I didn't find the answer, and sometimes putting the 多 after the measure word seems correct (e.g. 四块多美金 in this answer). I don't get it.

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I made a plot using Baidu search hits (e.g. “150多个小时” and “150个多小时”) comparing n多个小时 vs. n个多小时 (note the non-linear x-axis):

This is perhaps why I'm so confused about this.

I'm starting to feel like the logic here is that 多 represents an unspecified least significant digit. So something like 三块五 becomes 三块多, whereas 三十五块 becomes 三十多块. I'm not sure if this is correct.

Answer 1

Think of 多 after a measure word as "adding some more to the previous number, without making it to the next number".

5个多小时 = 5 hours plus some, but not quite 6 hours = between 5 and 6 hours

Note that 多 follows MW 个. It means between 五個小時 and 六個小時

150多个小时 = 150 hours plus some, but not quite 160 hours = between 150 and 160 hours

Here note that 多 follows MW 十 in 一百五十. It means between 一百五十個小時 and 一百六十個小時.

四块多美金 = between 4 and 5 USD

四十多块美金 = between 40 and 50 USD

Answer 2

The difference in placement of 多 between

I. 一/二/⋯⋯/九 + 十/百/千/萬⋯⋯ + 多 + 量詞

II. 一/二/⋯⋯/九 + 量詞 + 多

can be explained by the fact that 多 follows a 'unit' (including both base-10 placeholders and quantifiers), not the numbers 1-9 themselves.

Considering base-10 placeholders and quantifiers as similar syntactical entities is justified by the following example:

a. 一千多萬個小時

b. *一千萬多個小時

Here, 一千多萬 in example a. follows Rule I., where 萬 is syntactically similar to a quantifier. As in, literally, there are 'a thousand plus ten-thousands'.