To answer your first question, when you break the page down by month you are asking for server calls where month and page exist simultaneously, since you using page, every server call counted will be a page view. Remember that bounce is a visit with only 1 server call (and in this case that server call is a page view containing both January AND Page1 in order to be reported). Single Access is counting the number of visits in which Page1 AND January exist in a only one server call, which is this case is the same as a bounce (because page is only set on page views).
With Single Page Visit you're counting visits with more server calls than just the one page view containing Page1 AND January co-existing.
I tried duplicating the issue but i am getting the same values for the month after breaking it down. This would require a look inside your login company to check what is happening. I would request you to reach out to clientcare on this and get the issue resolved. Thanks!
For your second question, you should use single page visits in your calculation. Below is the difference between bounce rate, single access and single page visits and their comparison:
Bounces are defined as the number of visits to your site that contain a single image request. Custom links are included, meaning if a visit consists of a page view and a custom link, that visit is not considered a bounce.
Single Access is defined as the number of visits where there is only a single unique value seen through the entire visit. When the variable in question receives two unique values in a visit, it is no longer considered a Single Access. A Single Access visit can contain many page views and custom links, as long as the variable in question does not contain two unique values.
Single Page Visits is defined as the number of visits with a single page view. If a visit consists of a single page view and multiple custom link image requests, it is still considered a Single Page Visit. When a second page view is seen in a visit, it is no longer considered a Single Page Visit.
There are two types of Bounce Rate metrics available:
Bounce Rate available as a standard metric is the percentage of users who didn't make it past that single image request on your site. Bounce Rate applied as a calculated metric is defined by the end user, typically defined as Bounces divided by Entries.
Comparisons Single Access is always the highest, since it can include multiple page views while bounces and single page visits cannot.
The next highest is always Single Page Visits, since it can include custom link image requests while bounces cannot.
The lowest is always Bounces, since it only counts visits with a single image request.