On the cut to the forecast, the firm says that it is due to a large expected surplus over the near-term due to resilient Russian supplies. For some context, their previous forecast (on 6 June) was for Brent oil to average $111 per barrel this year and next citing a larger and sustained disruption to Russian supplies following sanctions by the EU.
Barclays does note that they "remain constructive on oil prices" though and I guess the forecast does underscore that sentiment, considering that Brent is now trading down to near $94.