reshape2 融化警告信息

I'm using melt and encounter the following warning message:
attributes are not identical across measure variables; they will be dropped

环顾四周，人们都提到这是因为变量是不同的类；然而，我的数据集并非如此.

以下是数据集:

test <- structure(list(park = structure(c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 
1L, 1L, 1L), .Label = c("miss", "piro", "sacn", "slbe"), class = "factor"), 
    a1.one = structure(c(3L, 1L, 3L, 3L, 3L, 3L, 1L, 3L, 3L, 
    3L), .Label = c("agriculture", "beaver", "development", "flooding", 
    "forest_pathogen", "harvest_00_20", "harvest_30_60", "harvest_70_90", 
    "none"), class = "factor"), a2.one = structure(c(6L, 6L, 
    6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L), .Label = c("development", 
    "forest_pathogen", "harvest_00_20", "harvest_30_60", "harvest_70_90", 
    "none"), class = "factor"), a3.one = structure(c(3L, 3L, 
    3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L), .Label = c("forest_pathogen", 
    "harvest_00_20", "none"), class = "factor"), a1.two = structure(c(3L, 
    3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L), .Label = c("agriculture", 
    "beaver", "development", "flooding", "forest_pathogen", "harvest_00_20", 
    "harvest_30_60", "harvest_70_90", "none"), class = "factor"), 
    a2.two = structure(c(6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 
    6L), .Label = c("development", "forest_pathogen", "harvest_00_20", 
    "harvest_30_60", "harvest_70_90", "none"), class = "factor"), 
    a3.two = structure(c(3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 
    3L), .Label = c("forest_pathogen", "harvest_00_20", "none"
    ), class = "factor")), .Names = c("park", "a1.one", "a2.one", 
"a3.one", "a1.two", "a2.two", "a3.two"), row.names = c(NA, 10L
), class = "data.frame")

struct 如下:

str(test)
'data.frame':   10 obs. of  7 variables:
 $ park  : Factor w/ 4 levels "miss","piro",..: 1 1 1 1 1 1 1 1 1 1
 $ a1.one: Factor w/ 9 levels "agriculture",..: 3 1 3 3 3 3 1 3 3 3
 $ a2.one: Factor w/ 6 levels "development",..: 6 6 6 6 6 6 6 6 6 6
 $ a3.one: Factor w/ 3 levels "forest_pathogen",..: 3 3 3 3 3 3 3 3 3 3
 $ a1.two: Factor w/ 9 levels "agriculture",..: 3 3 3 3 3 3 3 3 3 3
 $ a2.two: Factor w/ 6 levels "development",..: 6 6 6 6 6 6 6 6 6 6
 $ a3.two: Factor w/ 3 levels "forest_pathogen",..: 3 3 3 3 3 3 3 3 3 3

是因 for each 变量的级别数不同吗？那么，在这种情况下，我可以忽略警告信息吗？

要生成警告消息，请执行以下操作:

library(reshape2)
test.m <- melt (test,id.vars=c('park'))
Warning message:
attributes are not identical across measure variables; they will be dropped

谢谢