Observer Q about Papawarbucks hand 200/400

[doublebluff]

I dont really understand the bet on the river by PAPWARBUCKS anyone explain?

PokerStars Game #13047576760: Hold'em Limit ($200/$400) - 2007/11/04 - 22:01:05 (ET)
Table 'Pax' 6-max Seat #1 is the button
Seat 1: actnjunkie ($2643 in chips)
Seat 2: PapaWarbucks ($16330 in chips)
Seat 3: Baffa ($10800 in chips)
Seat 5: St1ckman ($38587 in chips)
PapaWarbucks: posts small blind $100
Baffa: posts big blind $200
*** HOLE CARDS ***
St1ckman: folds
actnjunkie: raises $200 to $400
PapaWarbucks: raises $200 to $600
Baffa: folds
actnjunkie: calls $200
*** FLOP *** [Td 4s 5s]
PapaWarbucks: bets $200
actnjunkie: calls $200
*** TURN *** [Td 4s 5s] [5c]
PapaWarbucks: bets $400
actnjunkie: calls $400
*** RIVER *** [Td 4s 5s 5c] [2d]
PapaWarbucks: bets $400
actnjunkie: calls $400
*** SHOW DOWN ***
PapaWarbucks: shows [Ac 7d] (a pair of Fives)
actnjunkie: shows [4h Ah] (two pair, Fives and Fours)
actnjunkie collected $3395 from pot
*** SUMMARY ***
Total pot $3400 | Rake $5
Board [Td 4s 5s 5c 2d]
Seat 1: actnjunkie (button) showed [4h Ah] and won ($3395) with two pair, Fives and Fours
Seat 2: PapaWarbucks (small blind) showed [Ac 7d] and lost with a pair of Fives
Seat 3: Baffa (big blind) folded before Flop
Seat 5: St1ckman folded before Flop (didn't bet)