I agree with you that the number of incorrect* off-topic - not about Ubuntu close votes has seemed particularly high of late. In addition to the two you mentioned, I've noticed that questions about Firefox and LibreOffice, questions asking for software recommendations, questions about text-processing, questions about dual-boot systems, questions about WSL, and questions that relate to hardware compatibility in some way, have received off-topic close votes and comments saying "this is not about Ubuntu" "this isn't specific to Ubuntu" "Hardware is off-topic here" "Software recommendations are off-topic here" "Windows is off-topic here", and so on.
* by incorrect, I mean, there is a reasonable consensus expressed in terms of upvoted posts here on meta, and a body of open, answered questions on the site, which upholds that questions of that kind are on-topic and acceptable here.
Eliah Kagan suggested a general explanation for this issue in chat
I think, vaguely speaking, review choices that involve knowing and correctly applying a policy (like, are questions about X on-topic according to the rules of the site?) can be made in two ways. Really there is something of a continuum between the two and this is about actions (which is my main point, the system does not adequately improve the actions that make up review). This is not a divide between the "good reviewers" and the "bad reviewers."
(A) by carefully checking that the policy is correct and, when there is any dispute or even a sign dispute or confusion might arise, providing a citation and
(B) carelessly translating personal feelings of whether or not a post is Ubuntuy enough into a review and--sometimes--an actual comment making unsupported claims about site policy.
And the problem is that (B) will always win out over (A) overall, because it is faster and easier and requires less energy and time and commitment to make review and commenting choices that fall under (B) than those that fall under (A).
I don't know how to fix this problem. But I would encourage people who want to review and use close votes to consider searching the help center and/or meta for policy when voting to close if they are in any doubt at all. When informing someone that their question is off-topic, it helps both the OP and other reviewers if we link to the policy stating that it is indeed off-topic. This also prevents us from carelessly making unsubstantiated statements and incorrectly voting to close (because we won't be able to find the link to the policy we think exists, if it doesn't exist :) ).
As a side note, I resent the implication in your question that people who apply wrong off-topic votes think they are "cleaning up". Have they said that's what they are doing, as your quote marks seem to suggest? Please don't discourage people from improving the site by suggesting that cleaning up is synonymous with careless close-voting!