Rain teams make use of Politoed's Drizzle support, making use of Water-type STAB attacks. Due to Rain and STAB, Water-type attacks hit for twice as much damage as normal (example: Scald hits for 80 BP normally. STAB multiplies it by 50% (+40). Drizzle adds another 50% (+40) for a total of 160 BP).
Your math is off. It's 80 * 1.5 * 1.5, which is more than twice normal. Although, I don't think non-Water types run Scald, to my knowledge...
80 * 2.25 = 180