drastic difference between half marathon and training runs

first pic is my splits from a 16k run i ran a week before my half marathon. second pic is my actual half marathon splits.

my splits for the first 16k during my race were significantly slower than my training run. during my training i could comfortably run past 10k without walking, and legs or feet aching, while maintaining a sub 6:20 average pace. yet during the half marathon i felt my performance declining after 10k despite running slower than average during the first 10k. i also spent the remaining KM doing jogging/walking intervals. i had never ran so slow ever since the beginning of my training in may.

im very confused about this. can anyone provide an explanation?