Union Attack Problems

Union Attack's text reads "Target 1 face-up monster you control; add up the total ATK of all other face-up Attack Position monsters you currently control, then during the Battle Phase of this turn only, the monster gains that much ATK...", however I have discovered this is inaccurate. It's text would imply that the ATK that the target monster gains is calculated upon Union Attack's activation though it seems that it is instead calculated as the Battle Phase begins. When I found this out I had 1 x Gravekeeper's Vassal, 2x Ojama Knight and Ojama Country on the field. With the effect of Ojama Country, both Ojama Knight's ATK were 2500 so I activated Union Attack and expected that Gravekeeper's Vassal gain a 5000 ATK boost. I then activated my prismatic Archfiend Palabyrinth for more duel assessment score which sent Ojama Country to the graveyard and my both Ojama Knight's ATK became 0 as expected. As both knights had 2500 ATK at the time of Union Attack's activation, the expected outcome was that my Gravekeeper's Vassal would gain 5000 ATK though nothing seemed to happen and so I believe that what actually happened is that my Gravekeeper's Vassal gained the ATK of my other monsters at the start of the battle phase contrary to the card's text. I cannot find any rulings online that would explain why this happens so I believe it is a bug.