Why can the construction "たと思ったら" be applied to the speaker himself?


> ゆめみ「実はすごい順調なんだよね、キャラデザは彩色まで終わって、もうキービジュアル入っているぐらいだし」
>
> 寿季「マジか。こないだキャラデザのラフを見 **たと思ったら** そこまでいってたとは……」

My grammar book says that the construction "たと思ったら" can’t be applied to the
speaker. So why is the usage in this sentence possible, in which the speaker
is talking about his own action?

For example, we can’t say 私は、うちに帰ったと思うとまた出かけた。

Could you please explain the phenomenon?




As for the sentence in question, キャラデザのラフを見た is about the state of _their_
work, the latter half of the sentence implies that the advance of the
production is quicker than the speaker expected.

Generally, X(だ)と思ったらY means the time between X and Y is shorter than expected.
As such, the time is not under the control of the speaker. That should explain
why 私はうちに帰ったと思うとまた出かけた is odd. The speaker decided to go out her/himself, so
there should be no surprise as regards the time span.

Probably the rule for the non-applicability of the construction should be
something along the lines of _Y not being the speaker's voluntary action_.

As an example, the following is possible:

  * 私はこないだ日本へ帰ってきたと思ったらまた来月アメリカに行くことになった
  * I just came back to Japan a short while ago, and I have to go to the USA next month.

Here Y is not the speaker's action but the situation where s/he has to go to
the US, and staying in Japan for a few months is shorter than the speaker
expected.

