ทำไมกำลัง rxGlm ไม่ส่งกลับผลลัพธ์เป็น glm เดียวกันหรือไม่

basictestdata <- data.frame(  Factor1 = as.factor(c(1,1,1,1,2,2,2,2)), 
 Factor2 = as.factor(c(1,1,2,2,1,1,2,2)), 
 Discount = c(1,2,1,2,1,2,1,2), 
 Exposure = c(24000, 40000, 7000, 14000, 7500, 15000, 2000, 5600), 
 PurePrem = c(46,32,73,58,48,25,220,30))

GLM.1 <- glm(PurePrem ~ Factor1 * Factor2 - 1, 
 family = tweedie(var.power = 1.5, link.power = 0), 
 data = basictestdata, weights = Exposure 
 , offset = log(Discount))

rxGlm.1 <- rxGlm(PurePrem ~ Factor1 * Factor2 - 1 + offset(log(Discount)), 
 family = rxTweedie(var.power = 1.5, link.power = 0), 
 data = basictestdata, fweights = "Exposure", dropFirst = TRUE, dropMain = FALSE)

coef(GLM.1) 
coef(rxGlm.1)