NIOS not reading the right FIFO value

If i change the synchronization of the fifo in the megawizard to show-ahead mode instead of normal mode (data becomes available before rdreq) i get the pairs

bash-3.1$ nios2-terminal
nios2-terminal: connected to hardware target using JTAG UART on cable
nios2-terminal: "USB-Blaster ", device 1, instance 0
nios2-terminal: (Use the IDE stop button or Ctrl-C to terminate)
loop start
0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:content of fifo:before DMA operation
0: 2
1: 4
2: 6
3: 8
4: a
5: c
6: e
7: 10
8: 12
9: 14
10: 16
11: 18
12: 1a
13: 1c
14: 1e
15: 20
16: 0
17: 0
18: 0
19: 0
20: 0
21: 0
22: 0
23: 0
24: 0
25: 0
26: 0
27: 0
28: 0
29: 0
30: 0
31: 0