The election ballot is open to all delegates with credentials regardless of General Assembly registration status. To vote on other business items you have to be registered for General Assembly.
No, there are more eligible voters for the board/commission elections: every society has a set number of delegate allotments; all may vote in these elections. Only delegates who attend GA, whether in person, on line, or business-only, may vote in the session-related elections (AIW, resolutions, etc.).
I am almost certain this is a quirk of the voting platform not handling multiwinner races correctly. All the ones where there are, for example, two winning candidates in two slots, the race gives each 50% of a total (voters*2), rather than 99% for each separately. The race with five candidates shows each with around 20%, etc.
See the page about how delegates are determined: How A Congregation’s Delegates are Determined. There are going to be regular shifts in the numbers as congregations and religious professionals get in touch with the UUA about out of date data or other data issues that may increase or decrease the total possible delegates (electors). There is no cut off to credential, so these numbers can continue to shift until the end of General Assembly.