I have an ESTA and entered the US on the 14th of March. Under ESTA I have to leave the country in 90 days from that date. During my travels I flew to Vancouver as well and stayed in Canada for 3 weeks. Recently I have flown back into the US. At the border control in Vancouver the officer apparently didn’t check my previous stamp in my passport and gave me a complete new one for 90 days, meaning I can now stay longer. Is my thinking correct? I also checked the I-94 website and it told me the same thing.
You can stay until your new "until" date. The visa waiver program regulations specify at 8 CFR 217.3(b) that an officer "may" readmit you for the balance of your previous period of admission. It is not required by regulation or statute. The text of the regulation:
(b) Readmission after departure to contiguous territory or adjacent island. An alien admitted to the United States under this part may be readmitted to the United States after a departure to foreign contiguous territory or adjacent island for the balance of his or her original Visa Waiver Pilot Program admission period if he or she is otherwise admissible and meets all the conditions of this part with the exception of arrival on a signatory carrier.