Before 1.3.0.4, completed fishing achievements would not grant you the Steam achievement and would reset when you restarted Terraria. So you get situations like this, for instance:
Playthrough|Fish caught|Good Little Slave counter (10)|Resets?|Fast and Fishious Counter (50)
1|8|8|No|8
2|4|10|Yes|12
3|6|6|No|18
4|5|10|Yes|23
5|7|7|No|30
And so on and so on.
So let's say, the last time you played Terraria
before 1.3.0.4. you got Good Little Slave (but not from Steam, because that was broken at the time) and got the Fast and Fishious counter to, say, 48. Then you updated to 1.3.0.4, and when you started Terraria, Good Little Slave reset again, and you had two fish to go until Fast and Fishious and ten until Good Little Slave, meaning that you would get Fast and Fishious first. It's not broken, just wonky resetting and all.