spark reduceByKeyAndWindow window have error interval - apache-spark

Question

I use stream 2.1 spark 1.6 scala program, the partial code is listed below.
Follow documentation saying that window size 3 and sliding size 2 and then windowed stream will appear at time 3 and time 5 but I got at time 4 and time 6 but windows size 3 and sliding size 1 is OK.
val inputData: mutable.Queue[RDD[String]] = mutable.Queue()
var outputCollector = new ArrayBuffer[(String, Int)]
val inputStream = ssc.queueStream(inputData)
val patternStream: DStream[(String, Int)] = inputStream.flatMap(line => {
line.replace(",", "").map(x => (x.toString(), 1))
})
val groupStream = patternStream.reduceByKeyAndWindow(_+_, _-_, Seconds(wt1), Seconds(st1))
inputStream.print()
patternStream.foreachRDD(rdd=>{
rdd.collect().foreach(print)
println("\n")
})
groupStream.foreachRDD(rdd => {
println("window stream")
rdd.filter(s => s._2>0).sortByKey().collect().foreach(i=> {outputCollector += (i)} )
})
window size 3 sliding size 1 it is ok
Time: 1000 ms
window stream (a,1)(b,1)(f,1)(g,1)
Time: 2000 ms
window stream (a,1)(b,1)(d,1)(e,1)(f,1)(g,1)
Time: 3000 ms
window stream (a,1)(b,1)(c,1)(d,2)(e,1)(f,1)(g,1)
Time: 4000 ms
window stream (a,1)(c,1)(d,3)(e,1)
Time: 5000 ms
window stream (a,1)(c,2)(d,3)
Time: 6000 ms
window stream (a,2)(c,2)(d,2)(f,1)(g,1)
window size 3 slice size 2
window stream appear in 2 4 6 have some error
Time: 1000 ms
Time: 2000 ms
window stream (a,1)(b,1)(d,1)(e,1)(f,1)(g,1)
Time: 3000 ms
############## no widowed streamhere
Time: 4000 ms
window stream (a,1)(c,1)(d,3)(e,1)
Time: 5000 ms
############ no widowed stream here
Time: 6000 ms
window stream (a,2)(c,2)(d,2)(f,1)(g,1)