用于分组和求和值的R Group by function给出问题[duplicate]

为什么这段代码:

letter = c("A","B","C","C","A")
product = c("Beef","Chicken","Beef","Beef","Beef")
value = c(10,20,40,10,5)
df <- data.frame(letter,product,value)

df1 = df %>% group_by(letter,product) %>% summarise(value = sum(value))

> df1
  value
1    85

而不是这个?

> df
  letter product value
1      A    Beef    15
2      B Chicken    20
3      C    Beef    50