There's huge gap between two results for iperf

Question

I used iperf -c FOO_NODE -t 120 -i 10 to test the network with the other node FOO_NODE, sometimes I get the result like:

[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.0 sec  3.75 MBytes  3.15 Mbits/sec
[  3] 10.0-20.0 sec  3.75 MBytes  3.15 Mbits/sec
[  3] 20.0-30.0 sec  3.62 MBytes  3.04 Mbits/sec
[  3] 30.0-40.0 sec  3.88 MBytes  3.25 Mbits/sec
[  3] 40.0-50.0 sec  4.25 MBytes  3.57 Mbits/sec
[  3] 50.0-60.0 sec  3.75 MBytes  3.15 Mbits/sec
[  3] 60.0-70.0 sec  4.00 MBytes  3.36 Mbits/sec
[  3] 70.0-80.0 sec  3.62 MBytes  3.04 Mbits/sec
[  3] 80.0-90.0 sec  3.25 MBytes  2.73 Mbits/sec
[  3] 90.0-100.0 sec  3.75 MBytes  3.15 Mbits/sec
[  3] 100.0-110.0 sec  4.38 MBytes  3.67 Mbits/sec
[  3] 110.0-120.0 sec  3.50 MBytes  2.94 Mbits/sec
[  3]  0.0-120.5 sec  45.6 MBytes  3.18 Mbits/sec

All are in range of 3~6

But sometimes I got the result like this:

[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.0 sec  54.0 MBytes  45.3 Mbits/sec
[  3] 10.0-20.0 sec  56.6 MBytes  47.5 Mbits/sec
[  3] 20.0-30.0 sec  58.2 MBytes  48.9 Mbits/sec
[  3] 30.0-40.0 sec  58.2 MBytes  48.9 Mbits/sec
[  3] 40.0-50.0 sec  59.4 MBytes  49.8 Mbits/sec
[  3] 50.0-60.0 sec  57.5 MBytes  48.2 Mbits/sec
[  3] 60.0-70.0 sec  58.0 MBytes  48.7 Mbits/sec
[  3] 70.0-80.0 sec  58.2 MBytes  48.9 Mbits/sec
[  3] 80.0-90.0 sec  58.8 MBytes  49.3 Mbits/sec
[  3] 90.0-100.0 sec  58.2 MBytes  48.9 Mbits/sec
[  3] 100.0-110.0 sec  57.6 MBytes  48.3 Mbits/sec
[  3] 110.0-120.0 sec  59.2 MBytes  49.7 Mbits/sec
[  3]  0.0-120.2 sec   694 MBytes  48.5 Mbits/sec

All are in range of 40~50.

And 3~6 happened more often. This's weird. What's the problem?

Update

I tested that from a node in ec2 to a node in other cloud server center. And I don't think there'll be much congestion cause both of them are less used.

Update2

The traceroute result is like this:

$ traceroute 54.223.144.68 -m 255
traceroute to 54.223.144.68 (54.223.144.68), 255 hops max, 60 byte packets
 1  172.17.5.128 (172.17.5.128)  1.071 ms  1.026 ms  1.005 ms
 2  10.255.153.28 (10.255.153.28)  0.877 ms  0.823 ms  0.818 ms
 3  * * *
 4  192.168.5.121 (192.168.5.121)  4.477 ms  4.724 ms  4.026 ms
 5  10.4.255.5 (10.4.255.5)  11.971 ms  10.970 ms  11.577 ms
 6  122.227.23.205 (122.227.23.205)  4.371 ms  1.970 ms 122.227.23.201 (122.227.23.201)  1.039 ms
 7  241.38.130.61.broad.jh.zj.dynamic.163data.com.cn (61.130.38.241)  6.853 ms 5.40.130.61.broad.jh.zj.dynamic.163data.com.cn (61.130.40.5)  3.888 ms 241.38.130.61.broad.jh.zj.dynamic.163data.com.cn (61.130.38.241)  6.978 ms
 8  202.97.68.173 (202.97.68.173)  37.155 ms 202.97.34.57 (202.97.34.57)  31.872 ms  31.864 ms
 9  * * *
10  182.222.120.106.static.bjtelecom.net (106.120.222.182)  34.517 ms  34.685 ms  34.488 ms
11  42.220.120.106.static.bjtelecom.net (106.120.220.42)  31.790 ms  36.454 ms  36.066 ms
12  54.222.1.67 (54.222.1.67)  33.093 ms 54.222.1.5 (54.222.1.5)  32.090 ms  32.059 ms
13  54.222.1.91 (54.222.1.91)  37.305 ms  38.982 ms  38.975 ms
14  * * *
15  * * *
16  * * *
17  * * *
18  * * *
19  * * *
20  * * *
21  * * *
22  * * *
23  * * *
24  * * *
25  * * *
26  * * *
27  * * *
28  * * *
29  * * *
30  * * *
31  * * *
32  * * *
...
254  * * *
255  * * *

All rest are *(255 is the max hop number), I just don't know why. Do I need to open some ports on ec2 server?