How do I interpret the mactime in case of aggregated data packets and block ack ?

In the sample capture output below following are my questions: 1) Packet #1 to #16 belong to same AMPDU but why does !16 have different mactime ? 2) The mactime increases from data pkt #16 to Ack #17, but after that decreases for the subsequent data pkt#18.

I have added the mactime delta column after calculation for reference.

Captures done using Wireshark on Ubuntu using ath9k driver corresponding to AR9287 chipset.

tshark -r pktFile.pcapng -N t -T fields -E separator=/t -e frame.number -e radiotap.mactime -e radiotap.ampdu.reference -e radiotap.ampdu.flags.last -e col.Protocol -e col.Info

1 2178550855 0 91215 0 UDP Source port: 38953 Destination port: 55225

2 2178550855 0 91215 0 UDP Source port: 38953 Destination port: 55225

3 2178550855 0 91215 0 UDP Source port: 38953 Destination port: 55225

4 2178550855 0 91215 0 UDP Source port: 38953 Destination port: 55225

5 2178550855 0 91215 0 UDP Source port: 38953 Destination port: 55225

6 2178550855 0 91215 0 UDP Source port: 38953 Destination port: 55225

7 2178550855 0 91215 0 UDP Source port: 38953 Destination port: 55225

8 2178550855 0 91215 0 UDP Source port: 38953 Destination port: 55225

9 2178550855 0 91215 0 UDP Source port: 38953 Destination port: 55225

10 2178550855 0 91215 0 UDP Source port: 38953 Destination port: 55225

11 2178550855 0 91215 0 UDP Source port: 38953 Destination port: 55225

12 2178550855 0 91215 0 UDP Source port: 38953 Destination port: 55225

13 2178550855 0 91215 0 UDP Source port: 38953 Destination port: 55225

14 2178550855 0 91215 0 UDP Source port: 38953 Destination port: 55225

15 2178550855 0 91215 0 UDP Source port: 38953 Destination port: 55225

16 2178553632 2777 91215 1 UDP Source port: 38953 Destination port: 55225

17 2178553846 214 802.11 802.11 Block Ack, Flags=........C

18 2178553697 -149 91216 0 UDP Source port: 38953 Destination port: 55225

19 2178553697 0 91216 0 UDP Source port: 38953 Destination port: 55225

20 2178553697 0 91216 0 UDP Source port: 38953 Destination port: 55225

21 2178553697 0 91216 0 UDP Source port: 38953 Destination port: 55225

22 2178553697 0 91216 0 UDP Source port: 38953 Destination port: 55225

23 2178553697 0 91216 0 UDP Source port: 38953 Destination port: 55225

24 2178553697 0 91216 0 UDP Source port: 38953 Destination port: 55225

25 2178553697 0 91216 0 UDP Source port: 38953 Destination port: 55225

26 2178553697 0 91216 0 UDP Source port: 38953 Destination port: 55225

27 2178553697 0 91216 0 UDP Source port: 38953 Destination port: 55225

28 2178553697 0 91216 0 UDP Source port: 38953 Destination port: 55225

29 2178553697 0 91216 0 UDP Source port: 38953 Destination port: 55225

30 2178553697 0 91216 0 UDP Source port: 38953 Destination port: 55225

31 2178553697 0 91216 0 UDP Source port: 38953 Destination port: 55225

32 2178553697 0 91216 0 UDP Source port: 38953 Destination port: 55225

33 2178556479 2782 91216 1 UDP Source port: 38953 Destination port: 55225

34 2178556693 214 802.11 802.11 Block Ack, Flags=........C

35 2178556543 -150 91217 0 UDP Source port: 38953 Destination port: 55225