It's because the visa is Non-Imm O for Retirement. Initially issued for 90 days, but with a specific view to using that time to find a home address, open a bank account and allow the money to sit for 2 months, before applying for a 12 month extention.
Jim, starting again in the UK with a new 90 day visa, still means you have to have 800k in a Thai bank for 2 months before they will issue a 12 month exemption. I sympathise with the problem of not being able to use the monthly income option. I strongly advise you seek help from an agent to get your renewal
It is only difficult to open a bank account if you don't qualify. You need a long term non-Imm visa. Apply through the Thai embassy on-line. If you qualify they will grant your visa for 90 days. Arrive in Thailand, find somewhere to live (hotel for a few weeks will suffice) and make sure they have reported you living there. Then visit Immigration and ask for a Residency certificate to open a bank account. They will even address it to the branch of the bank you'd like. Open the account with the minimum funds required for your type of visa. Once the funds have been in there for the required period, usually 2 months, apply for the 12 month extension
There is no limit, except the end of your visa. If it expired without you being here to extend it you would have to start again. Meanwhile wherever you found the reference to 45 days is nonsense
If you want to be sure go and ask Immigration. The people there are not your enemy, but that is how you act. Are you sure you're happy in Thailand, if you're so suspicious of Thai authority figures?