有关 strings.Builder 内存分配问题?

写到第十一个的时候，就发生了内存分配。为什么会这样？

func numAllocs(fn func()) uint64 {
    defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(1))
    var m1, m2 runtime.MemStats
    runtime.ReadMemStats(&m1)
    fn()
    runtime.ReadMemStats(&m2)
    return m2.Mallocs - m1.Mallocs
}
func TestStrings_numAllocs(t *testing.T) {
    // Builder’s inner slice’s capacity: 10
    // Builder’s inner slice’s length: 5
    // current_capacity * 2 + n
    var sql strings.Builder
    sql.Grow(10) // capacity should 30
    allocs := numAllocs(func() {
        for i := 0; i < 11; {
            sql.WriteByte('a')
            i++
        }
        _ = sql.String()
    })
    if allocs > 0 {
        t.Fatalf("got %d alloc(s); want 0", allocs)
    }
}