Okay. After more extensive testing here are the results:
Testing location: Dalaran Sewers
1.2 * ((500[base] / 80[level] * 100) + 3600[spell power] / 1.064) = 4810 absorb == Correct
Testing location: Outside Silvermoon City
1.2 * ((500[base] / 80[level] * 100) + 3600[spell power] / 1.064) = 4810 absorb == Correct
Testing location: Icecrown Citadel (25% buff)
1.25 * ( 1.2 * ((500[base] / 80[level] * 100) + 3600[spell power] / 1.064)) = 6012 absorb == Incorrect :: Result 4810 absorb
Testing location: Icecrown Citadel (0% buff)
1.2 * ((500[base] / 80[level] * 100) + 3600[spell power] / 1.064) = 4810 absorb == Correct
Testing location: Trial of the Crusader
1.2 * ((500[base] / 80[level] * 100) + 3600[spell power] / 1.064) = 4810 absorb == Correct
Conclusion: From this testing is has resulted that either Sacred Shield has received a 25% in the absorb amount or the spell is currently bugged and is
always regardless of location receiving the 25% buff from Icecrown Citadel. Until the buff increases to 30% testing can not be conducted to disprove either.
EDIT: If the bug is currently with with 25% from Icecrown Citadel then the spells absorb amount is:
([Spell_Power] * 0.75 + 500)